hwmon: (pmbus/lm25066) Report VAUX as vmon

So far the driver reported the voltage on VAUX as "vout2". This was not
entirely appropriate as it is not an output voltage, and complicates
the code. Use the new virtual "VMON" register set and report the voltage
as "vmon" instead.

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