CONFIG_ARCH_MX51 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_ARCH_MX51 (not configurable)

configname: CONFIG_ARCH_MX51

Linux Kernel Configuration
└─>ARCH_MX51
In linux kernel since version 2.6.34 (release Date: 2010-05-16)  
depends
CONFIG_ARCH_MX5

is depended by
CONFIG_SOC_IMX51