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

i.MX51 support

modulename: imx51-dt.ko

configname: CONFIG_MACH_IMX51_DT

Linux Kernel Configuration
└─>Freescale i.MX family
└─>i.MX51 support
In linux kernel since version 3.1 (release Date: 2011-10-24)  
This enables support for Freescale i.MX51 processor

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