dt-bindings: pinctrl: mt8186: add pinctrl file and binding document

1. This patch adds pinctrl file for mt8186.
2. This patch adds mt8186 compatible node in binding document.
Signed-off-by: Guodong Liu <guodong.liu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220216032124.28067-2-guodong.liu@mediatek.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>