[Technical Field of the Invention] [0001] The present invention relates to a method of manufacturing a mechanical part and a mechanical part. Priority is claimed on Japanese Patent Application No. 2015-7912, filed on January 19, 2015 and Japanese Patent Application No. 2015-124050, filed on June 19, 2015, the contents of which are incorporated herein by reference. [Related Art] [0002] In a metal mechanical part, for example, in a drive plate or a ring gear which is a component of a vehicular automatic transmission, a plate thickness of the ring gear is thicker than a plate thickness of the drive plate in order to secure tooth strength of the ring gear. Accordingly, in general, after the drive plate and the ring gear are separately manufactured, the ring gear is welded to the drive plate to manufacture a single toothed part having both structures. Moreover, as described in Patent Documents 1 and 2 below, a method which manufactures a single toothed part having both structures of the drive plate and the ring gear from, a single material is known. In this method, after an annular wall portion is formed along an outer circumference of a single material (for example, a circular metal plate) by drawing the material, a toothed portion (a portion corresponding to the ring gear) is formed in the circumferential direction of the wall portion while the wall portion is thickened by forging the wall portion using a toothed die. [Prior Art Document] [Patent Document] [0003] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H7-256377 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H9-222158 [Disclosure of the Invention] [Problems to be Solved by the Invention] [0004] In the method which manufactures the single toothed part by welding the drive plate and the ring gear, there is a problem that a manufacturing cost increases since the number of manufacturing steps increases, and there is a problem that it is difficult to secure assembling accuracy at the time of the weldings As described in Patent Documents 1 and 2, in the method of manufacturing the toothed part having both structures of the drive plate and the ring gear from the single material by forging, since the thickening amount (thickness) of the toothed portion (ring gear portion) is limited by the plate thickness of the material, there is a limitation to increase the thickness of the toothed portion. In addition, in order to secure strength of the toothed portion, it is necessary to increase a thickening ratio of the toothed portion. However, if the thickening ratio increases, a folding failure is likely to occur in a root of the toothed portion. [0005] For example, in the toothed part having both structures of the drive plate and the ring gear, a target thickness of the toothed portion (ring gear portion) and target thicknesses of other disk-shaped parts (drive plate portion) are greatly different from each other. Specifically, the thickness of the drive plate portion may be the same as the plate thickness of the material. However, the thickness of the toothed portion is required to be several times the plate thickness of the material so as to secure the strength. In addition, the thickness of the toothed portion indicates a length of the toothed portion in a radial direction (a direction orthogonal to the plate thickness direction of the material) of the toothed part. [0006] However, as described above, in the case where the toothed part having both structures of the drive plate and the ring gear is manufactured by forging the single material, since the thickness of the toothed portion is limited by the plate thickness of the material (that is, there is a limitation in the thickening ratio), it is difficult to set the thickness of the toothed portion to the target thickness (several times the plate thickness of the material). Accordingly, in the related art, it is required to develop a technology which can manufacture a mechanical part which includes portions each having significantly different target thickness, by forging a single material. [0007] Particularly, in recent years, as a method of manufacturing a mechanical part, which requires high dimension accuracy and high strength such as a toothed part for an automatic transmission, from a single material, cold forging has attracted attention. Compared to hot forging, the cold forging has advantages such as obtaining a mechanical part having high accuracy and high strength, decreasing a manufacturing cost, increasing yield, or the like. Accordingly, if it is possible to develop a technology capable of manufacturing a mechanical part which includes portions each having significantly different target thickness, by forging a single material, by combining this technology and the cold forging, it is possible to provide an ideal technology capable of maximizing advantages (high precision and high strengthening) on the quality of the mechanical part and advantages (low cost and high yield) on a manufacturing process. [0008] The present invention is made in consideration of the above-described circumstances, and an object thereof is to provide a method of manufacturing a mechanical part capable of manufacturing a mechanical part which includes portions each having significantly different target thickness, by forging a single material, and a mechanical part which is manufactured by the manufacturing method. [Means for Solving the Problem] [0009] The present invention adopts the following means in order to solve the problems and to achieve the object. (1) According to an aspect of the present invention, there is provided a method of manufacturing a mechanical part, including: a first process of forming, by performing a folding processing to an end portion of the material, a portion to be processed having a structure, in which a plurality of layers respectively having a thickness corresponding to a plate thickness of a material overlap each other, in the material such that a plate thickness direction of the layer is orthogonal to a plate thickness direction of the material; and a second process of changing, by performing a forging processing to the portion to be processed, a shape of the portion to be processed to a target shape while press-welding the layers of the portion to be processed to each other by plastic deformation. [0010] (2) In the method of manufacturing a mechanical part according to (1), in the first process, the portion to be processed having a structure in which two layers or three layers overlap each other may be formed in the material. [0011] (3) In the method of manufacturing a mechanical part according to (1) or (2), the first process may include: forming, by performing the folding processing to the end portion of the material, a pre-bent portion in which a part having a thickness corresponding to the plate thickness of the material is continuously formed in a zigzag shape, and forming, by adding an external force to the pre-bent portion such that the parts of the pre-bent portion overlap each oilier, the portion to be processed having a structure in which a plurality of parts overlap each other as the layers. [0012] (4) In the method of manufacturing a mechanical part according to any one of (1) to (3), the first process may include thickening the portion to be processed in the plate thickness direction of the layer by adding a pressing force to the portion to be processed in the plate thickness direction of the material. [0013] (5) In the method of manufacturing a mechanical part according to (4), when a length of the portion to be processed in the plate thickness direction of the layer is defined as a thickness of the portion to be processed, a thickness of the portion to be processed before the thickening is defined as TO, and a thickness of the portion to be processed after the thickening ends is defined as Tel, in the thickening, a thickening amount of the portion to be processed may be controlled such that a thickening ratio Tra which is obtained by dividing the thickness Tel by the thickness TO satisfies the following Conditional Expression (1). 1.0