hwmon: (sbrmi) Add Documentation

- Add documentation for SB-RMI module

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Akshay Gupta <Akshay.Gupta@amd.com>
Signed-off-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
Link: https://lore.kernel.org/r/20210726133615.9709-2-nchatrad@amd.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>