dt-bindings: gpio: pca953x: Document interrupts, update example

The device can optionally supply an interrupt, hence document that.

Add required GPIO properties to the example (extracted from a patch by
Wolfram Sang).

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/gpio/gpio-pca953x.txt

···
  - #interrupt-cells: if to be used as interrupt expander.
 
 Optional properties:
+ - interrupts: interrupt specifier for the device's interrupt output.
  - reset-gpios: GPIO specification for the RESET input. This is an
 		active low signal to the PCA953x.
  - vcc-supply:	power supply regulator.
···
 		reg = <0x20>;
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_pca9505>;
+		gpio-controller;
+		#gpio-cells = <2>;
 		interrupt-parent = <&gpio3>;
 		interrupts = <23 IRQ_TYPE_LEVEL_LOW>;
 	};