hwmon: (pmbus/ltc2978) Add support for LTC3880 to LTC2978 driver

The LTC3880 PMBus command set is comparable to LTC2978. Add support for it
to the LTC2978 driver.

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