hwmon: (pmbus) Add support for additional voltage sensor

Some PMBus chips support monitoring an additional non-standard voltage. While
this voltage can in many cases be supported by simulating an additional sensor
page, this does not work in all cases. Specifically, it is problematic if the
data format is linear and the voltage is reported in LINEAR11 format. Since
output voltages use LINEAR16, and the exponent for LINEAR16 data is chip-wide
and fixed, this can result in overflows.

To solve this problem, add support for an additional virtual input voltage,
call it 'vmon', and treat this voltage as input voltage (which, when the chip
supports linear data format, uses LINEAR11).

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