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

Freescale MXS-based

modulename: mach-mxs.ko

and/or

configname: CONFIG_ARCH_MXS

ARM system type
└─>Freescale MXS-based
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
Support for Freescale MXS-based family of processors


depends
 or CONFIG_ARCH_MULTI_V5

is selected by
CONFIG_DRM_MXSFB