dt-bindings: power: supply: Add Richtek RT9471 battery charger

Add bindings for the Richtek RT9471 I2C controlled battery charger.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
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>