dt-bindings: backlight: Add MediaTek MT6370 backlight

Add MT6370 backlight binding documentation.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220830034042.9354-5-peterwu.pub@gmail.com