dt-bindings: mfd: Add MediaTek MT6370

Add MediaTek MT6370 binding documentation.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220805070610.3516-7-peterwu.pub@gmail.com