CONFIG_SERIAL_RM9000 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips
default or selected kernelversion does not have config value CONFIG_SERIAL_RM9000.
Result is shown for kernelversion 6.2.5

CONFIG_SERIAL_RM9000 (not configurable)

configname: CONFIG_SERIAL_RM9000

Linux Kernel Configuration
└─>SERIAL_RM9000
In linux kernel since version 2.6.19 (release Date: 2006-11-29)  
is selected by
CONFIG_SERIAL_8250_RM9K