CONFIG_ARCH_MXC is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Freescale MXC/iMX-based

configname: CONFIG_ARCH_MXC

ARM system type
└─>Freescale MXC/iMX-based
In linux kernel since version 2.6.23 (release Date: 2007-10-09)  
Support for Freescale MXC/iMX-based family of processors