dt-bindings: pinctrl: airoha: Add EN7581 pinctrl

Introduce device-tree binding documentation for Airoha EN7581 pinctrl
controller.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/20241023-en7581-pinctrl-v9-2-afb0cbcab0ec@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>