CONFIG_MACH_IMX31_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support i.MX31 platforms from device tree

modulename: imx31-dt.ko

configname: CONFIG_MACH_IMX31_DT

Linux Kernel Configuration
└─>Support i.MX31 platforms from device tree
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Include support for Freescale i.MX31 based platforms
using the device tree for discovery.

source code:
arch/arm/mach-imx/imx31-dt.c