power: supply: rt9471: Add Richtek RT9471 charger driver

Add support for the RT9471 3A 1-Cell Li+ battery charger.

The RT9471 is a highly-integrated 3A switch mode battery charger with
low impedance power path to better optimize the charging efficiency.

Co-developed-by: Alina Yu <alina_yu@richtek.com>
Signed-off-by: Alina Yu <alina_yu@richtek.com>
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>