hwmon: (sbtsi) Add documentation

Document the SB-TSI sensor interface driver.

Signed-off-by: Kun Yi <kunyi@google.com>
Link: https://lore.kernel.org/r/20201211215427.3281681-3-kunyi@google.com


[groeck: Added SPDX license identifier, same as source]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>