hwmon: (pmbus) Add support for MAX20751

MAX20751 is a multiphase power controller with internal buck converter.
It uses VR12.0 to report the output voltage. This requires an explicit
driver, since the VR version can not be auto-detected.

The chip supports a manufacturer specific command to fine-tune the output
voltage.  This command is not currently supported.

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