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