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

RDA Micro SoCs

configname: CONFIG_ARCH_RDA

Linux Kernel Configuration
└─>RDA Micro SoCs
In linux kernel since version 5 (release Date: 2019-03-03)  
This enables support for the RDA Micro 8810PL SoC family.