hwmon: (pmbus) Support reading and writing of word registers in device specific code

Some PMBus devices use non-standard registers for some of the sensors and/or
limits. To support such devices, add code to support reading and writing of word
size registers in device specific code.

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