hwmon: (corsair-psu) add support for reading PWM values and mode

Add support for reading PWM values and mode,
and update documentation accordingly.

Signed-off-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/ZJSASByXpzoZ0XyH@monster.localdomain


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