CONFIG_ARCH_MX1 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

MX1-based

modulename: iomux-mx1-mx2.ko

configname: CONFIG_ARCH_MX1

MXC/iMX Base Type
└─>MX1-based
In linux kernel since version 2.6.29 (release Date: 2009-03-23)  
This enables support for systems based on the Freescale i.MX1 family

source code:

is selected by
CONFIG_MACH_SCB9328

is depended by
CONFIG_SOC_IMX1