dt-bindings: pinctrl: mediatek: add support for mt8188

Add the pinctrl header file on MediaTek mt8188.
Add the new binding document for pinctrl on MediaTek mt8188.

Signed-off-by: Hui.Liu <hui.liu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20220818075012.20880-2-hui.liu@mediatek.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>