dt-bindings: pinctrl: add binding for MT7988 SoC

This adds bindings for MT7988 pinctrl driver.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/20241217085435.9586-4-linux@fw-web.de
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: frank-w