docs: hwmon: Document max127 driver

Add documentation for the max127 hardware monitoring driver.

Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201123185658.7632-3-rentao.bupt@gmail.com


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