pinctrl: mt8183: add DT binding document

The commit adds mt8183 compatible node in binding document.

Signed-off-by: Zhiyong Tao <zhiyong.tao@mediatek.com>
Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>