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

and/or

irq-mxs.ko

and/or

mxs_timer.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

source code:
arch/arm/mach-mxs/mach-mxs.c


is selected by
CONFIG_PWM_MXS