dt-bindings: arm,cci-400: Add interrupt controller to example

In order to parse the 'interrupts' an interrupt provider node is needed.
That is because the example is a full example (starting with root node)
and on those we don't guess and generate a fake provider.
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220126231146.1632819-1-robh@kernel.org

Documentation/devicetree/bindings/arm/arm,cci-400.yaml

@@ -119,6 +119,11 @@ examples:
           arm,hbi = <0x249>;
           interrupt-parent = <&gic>;
 
+          gic: interrupt-controller {
+              interrupt-controller;
+              #interrupt-cells = <3>;
+          };
+
           /*
            * This CCI node corresponds to a CCI component whose control
            * registers sits at address 0x000000002c090000.