power: supply: max77976: add Maxim MAX77976 charger driver

Add support for the MAX77976 3.5/5.5A 1-Cell Li+ Battery Charger.

This is a simple implementation enough to be used as a simple battery
charger without OTG and boost.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>