dt-bindings: pwm: airoha: Add EN7581 pwm

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

Co-developed-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/20241023-en7581-pinctrl-v9-3-afb0cbcab0ec@kernel.org


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