dt-bindings: pinctrl: airoha: Add EN7581 pinctrl

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

Reviewed-by: Rob Herring (Arm) <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno <[email protected]>
Signed-off-by: Lorenzo Bianconi <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Linus Walleij <[email protected]>

Author: LorenzoBianconi