CONFIG_SOC_IMX51 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_SOC_IMX51 (not configurable)

modulename: mach-imx51.ko

configname: CONFIG_SOC_IMX51

Linux Kernel Configuration
└─>Freescale i.MX family
└─>SOC_IMX51
In linux kernel since version 2.6.38 (release Date: 2011-03-14)