dt-bindings: PCI: socionext,uniphier-pcie: Add missing child interrupt controller

The Socionext interrupt controller internal to the the PCI block isn't
documented which causes warnings when unevaluatedProperties check is
also fixed. Add the 'interrupt-controller' child node and properties and
fixup the example so that interrupt properties can be parsed.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20220525210117.2489333-1-robh@kernel.org