CONFIG_ARCH_MX5 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

MX5-based

configname: CONFIG_ARCH_MX5

Freescale CPU family:
└─>MX5-based
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_CPU_V7