The LTC3880 PMBus command set is comparable to LTC2978. Add support for it
to the LTC2978 driver.
Signed-off-by: NGuenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: NRobert Coulson <robert.coulson@ericsson.com>

Provide explicit driver for LTC2978 to enable support for minimum and peak
attributes. Remove ltc2978 chip id from generic pmbus driver.
Signed-off-by: NGuenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: NRobert Coulson <robert.coulson@ericsson.com>