CONFIG_SOC_AT91RM9200 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

AT91RM9200

modulename: at91rm9200.ko

configname: CONFIG_SOC_AT91RM9200

Linux Kernel Configuration
└─>Atmel AT91 System-on-Chip
└─>AT91RM9200
In linux kernel since version 3.1 (release Date: 2011-10-24)  
source code:
/arch/arm/mach-at91/