hwmon: (pmbus) Add get_voltage/set_voltage ops

The pmbus core does not have operations for getting or setting voltage.
Add functions get/set voltage for the dynamic regulator framework.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20220503104631.3515715-5-marten.lindahl@axis.com


[groeck: cosmetic alignment / empty line fixes]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>