gpio: adnp: dt: Reference generic interrupt binding

Instead of having to duplicate the description of the properties needed
for interrupt support, reference the new standard document.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
[Minor spelling correction]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -11,6 +11,10 @@ Required properties:
 - gpio-controller: Marks the device as a GPIO controller
 - nr-gpios: The number of pins supported by the controller.
 
+The GPIO expander can optionally be used as an interrupt controller, in
+which case it uses the default two cell specifier as described in
+Documentation/devicetree/bindings/interrupt-controller/interrupts.txt.
+
 Example:
 
 	gpioext: gpio-controller@41 {