default or selected kernelversion does not have config value CONFIG_SND_OMAP_SOC_RX51.
Result is shown for kernelversion 6.9.9

SoC Audio support for Nokia N900 (RX-51)

modulename: snd-soc-rx51.ko

configname: CONFIG_SND_OMAP_SOC_RX51

Linux Kernel Configuration
└─>Device Drivers
└─>Sound card support
└─>Advanced Linux Sound Architecture
└─>ALSA for SoC audio support
└─>GPIO Support
└─>SoC Audio support for Nokia N900 (RX-51)
In linux kernel since version 2.6.35 (release Date: 2010-08-01)  
Say Y if you want to add support for SoC audio on Nokia N900
cellphone.

source code:
sound/soc//omap/