dt-bindings: pinctrl: add #interrupt-cells to apple,pinctrl

The GPIO/pinctrl hardware can act as an interrupt-controller, so add
the #interrupt-cells property to the binding.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Acked-by: Rob Herring <robh@kernel.org>