CONFIG_MACH_IMX51_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support i.MX51 platforms from device tree

modulename: imx51-dt.ko

configname: CONFIG_MACH_IMX51_DT

Linux Kernel Configuration
└─>Support i.MX51 platforms from device tree
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Include support for Freescale i.MX51 based platforms
using the device tree for discovery

source code:
arch/arm/mach-imx/imx51-dt.c