gpio: zynq: Document interrupt-controller DT binding

HW and driver support the GPIO as interrupt-controller.
Document that in the DT binding.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -12,6 +12,13 @@ Required properties:
 - interrupts		: Interrupt specifier (see interrupt bindings for
 			  details)
 - interrupt-parent	: Must be core interrupt controller
+- interrupt-controller	: Marks the device node as an interrupt controller.
+- #interrupt-cells 	: Should be 2.  The first cell is the GPIO number.
+			  The second cell bits[3:0] is used to specify trigger type and level flags:
+			      1 = low-to-high edge triggered.
+			      2 = high-to-low edge triggered.
+			      4 = active high level-sensitive.
+			      8 = active low level-sensitive.
 - reg			: Address and length of the register set for the device
 
 Example:
@@ -22,5 +29,7 @@ Example:
 		gpio-controller;
 		interrupt-parent = <&intc>;
 		interrupts = <0 20 4>;
+		interrupt-controller;
+		#interrupt-cells = <2>;
 		reg = <0xe000a000 0x1000>;
 	};