dt-bindings: gpio: altera: correct interrupt-cells

commit 3a21c3ac upstream.

update documentation to correctly state the interrupt-cells to be 2.

Cc: stable@vger.kernel.org
Fixes: 4fd9bbc6

 ("drivers/gpio: Altera soft IP GPIO driver devicetree binding")
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>