hw/arm/virt: Fix devicetree warning about the GIC node

The GICv3 bindings requires a #msi-cells property for the ITS node. Fix
the corresponding dt-validate warning:

  interrupt-controller@8000000: msi-controller@8080000: '#msi-cells' is a required property
  From schema: linux/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 20220927100347.176606-3-jean-philippe@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>