dt-bindings: power: supply: Add MediaTek MT6370 Charger

Add MediaTek MT6370 Charger binding documentation.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>