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

Socionext Milbeaut SoCs

configname: CONFIG_ARCH_MILBEAUT

Linux Kernel Configuration
└─>Socionext Milbeaut SoCs
In linux kernel since version 5.1 (release Date: 2019-05-05)  
This enables support for Socionext Milbeaut SoCs