docs: hwmon: add max31760 documentation

Adding documentation for max31760 fan speed controller

Signed-off-by: Ibrahim Tilki <Ibrahim.Tilki@analog.com>
Link: https://lore.kernel.org/r/20220910171945.48088-3-Ibrahim.Tilki@analog.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>