CONFIG_PPC_MPC512x is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_PPC_MPC512x (not configurable)

modulename: mpc5xxx_clocks.ko

configname: CONFIG_PPC_MPC512x

Linux Kernel Configuration
└─>PPC_MPC512x
In linux kernel since version 2.6.25 (release Date: 2008-04-16)  
source code:
/arch/powerpc/sysdev/mpc5xxx_clocks.c

depends
 or CONFIG_6xx