CONFIG_M523x is not available for thedefaultarchitecture x86.
Result is shown for architecture m68k

MCF523x

modulename: m523x.ko

configname: CONFIG_M523x

Linux Kernel Configuration
└─>MCF523x
In linux kernel since version 2.6.14 (release Date: 2005-10-27)  
Freescale Coldfire 5230/1/2/4/5 processor support

source code:
arch/m68k/platform/coldfire/


is selected by
CONFIG_FEC