hwmon: (pmbus) Add support for peak attributes

Most PMBus devices provide manufacturer specific commands to read low and/or
high peak values for some or all of its sensors.

To support providing those values as lowest/highest attributes to the user,
introduce virtual PMBus commands. Those commands reside outside the normal
command set and have to be implemented in device specific code, which map the
virtual commands to device specific commands.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>