CONFIG_ARCH_MX51 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_ARCH_MX51.
Result is shown for kernelversion 6.2.5

i.MX51

configname: CONFIG_ARCH_MX51

Freescale CPU family:
└─>i.MX51
In linux kernel since version 2.6.34 (release Date: 2010-05-16)  
This enables support for systems based on the Freescale i.MX51 family

selects
CONFIG_ARCH_MX51_SUPPORTED

is selected by
CONFIG_SOC_IMX53