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

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>

Documentation/devicetree/bindings/gpio/gpio-altera.txt

@@ -9,8 +9,9 @@ Required properties:
   - The second cell is reserved and is currently unused.
 - gpio-controller : Marks the device node as a GPIO controller.
 - interrupt-controller: Mark the device node as an interrupt controller
-- #interrupt-cells : Should be 1. The interrupt type is fixed in the hardware.
+- #interrupt-cells : Should be 2. The interrupt type is fixed in the hardware.
   - The first cell is the GPIO offset number within the GPIO controller.
+  - The second cell is the interrupt trigger type and level flags.
 - interrupts: Specify the interrupt.
 - altr,interrupt-type: Specifies the interrupt trigger type the GPIO
   hardware is synthesized. This field is required if the Altera GPIO controller
@@ -38,6 +39,6 @@ gpio_altr: gpio@ff200000 {
 	altr,interrupt-type = <IRQ_TYPE_EDGE_RISING>;
 	#gpio-cells = <2>;
 	gpio-controller;
-	#interrupt-cells = <1>;
+	#interrupt-cells = <2>;
 	interrupt-controller;
 };