CONFIG_ARCH_MXS is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
Freescale MXS (i.MX23, i.MX28) support
modulename: mach-mxs.ko
configname: CONFIG_ARCH_MXS
Linux Kernel Configuration
└─>Freescale MXS (i.MX23, i.MX28) support
In linux kernel since version 2.6.38 (release Date: 2011-03-14)
Support for Freescale MXS-based family of processors