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

CONFIG_SOC_IMX51 (not configurable)

configname: CONFIG_SOC_IMX51

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

is selected by
CONFIG_PINCTRL_IMX51