dt-bindings: pinctrl: mt2712: add binding document

The commit adds mt2712 compatible node in binding document.

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