Gain,Offset with dbus-sensors values

[Patrick Venture]

James;

We have some sensors that need a gain and offset value set in the json
file, for PSU sensors, but only vout/vin or some specific ones, and
not applied to all sensors.

Akin to: https://github.com/openbmc/phosphor-hwmon/blob/2227bd52c5bb8d62dcfe33b58d65515d21e9be17/sensor.cpp#L59

Also, is there a way to customize the very verbose suffixes PSUsensor applies
such as "Temperature" and "Output_Power".  With IPMI we're only
granted 16 characters for the name and ipmid uses the last part of the
dbus path for the name.

Perhaps ipmid should allow overriding the name value.

Patrick