CONFIG_MACH_UX500_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_MACH_UX500_DT.
Result is shown for kernelversion 6.5.1

Generic U8500 support using device tree

configname: CONFIG_MACH_UX500_DT

Linux Kernel Configuration
└─>Ux500 target platform (boards)
└─>Generic U8500 support using device tree
In linux kernel since version 3.1 (release Date: 2011-10-24)  
selects
CONFIG_USE_OF