dt-bindings: pinctrl: mt8192: add binding document

The commit adds mt8192 compatible node in binding document.

Signed-off-by: Zhiyong Tao <zhiyong.tao@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200817001702.1646-3-zhiyong.tao@mediatek.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>