hwmon: (pmbus/ltc2978) Explicit driver for LTC2978

Provide explicit driver for LTC2978 to enable support for minimum and peak
attributes. Remove ltc2978 chip id from generic pmbus driver.

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