dt-bindings: pinctrl: update bindings for MT7629 SoC

This updates bindings for MT7629 pinctrl driver.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Sean Wang <sean.wang@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>