hwmon: (dell-smm) Add SMM interface documentation

Document the SMM interface as requested by Pali Rohar.
Since Dell does not offer any offical documentation
regarding the SMM interface, the necessary information
was extracted from the dell_smm_hwmon driver and other
sources.

Suggested-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20220215191113.16640-7-W_Armin@gmx.de


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