hwmon: (pmbus/zl6100) Add support for VMON/VDRV

Some of the ZL6100 compatible chips support monitoring a separate voltage pin,
VMON (ZL2004) or VDRV (ZL91xx). Report it as in2 / vmon.

The chips support implicit warning limits for VMON/VDRV, as percentage of the
respective critical voltage. Support by reading/writing the critical voltages
and calculating the associated warning voltages.

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