CONFIG_MPC512x_GENERIC is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Generic support for simple MPC512x based boards

modulename: mpc512x_generic.ko

configname: CONFIG_MPC512x_GENERIC

Linux Kernel Configuration
└─>Generic support for simple MPC512x based boards
In linux kernel since version 3.1 (release Date: 2011-10-24)  
This option enables support for simple MPC512x based boards
which do not need custom platform specific setup.

Compatible boards include: Protonic LVT base boards (ZANMCU
and VICVT2), Freescale MPC5125 Tower system.

source code: