docs: hwmon: add docs for the NXP MC34VR500 PMIC

Add documentation for the NXP MC34VR500 PMIC.

Signed-off-by: Mario Kicherer <dev@kicherer.org>
Link: https://lore.kernel.org/r/20230118123019.3041303-3-dev@kicherer.org


[groeck: Added to index]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>