default or selected kernelversion does not have config value CONFIG_SERIAL_8250_RM9K.
Result is shown for kernelversion 6.2.5

Support for MIPS RM9xxx integrated serial port

configname: CONFIG_SERIAL_8250_RM9K

Linux Kernel Configuration
└─>Device Drivers
└─>Character devices
└─>Serial drivers
└─>Support for MIPS RM9xxx integrated serial port
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Selecting this option will add support for the integrated serial
port hardware found on MIPS RM9122 and similar processors.
If unsure, say N.