hw/arm/virt: Fix devicetree warnings about the virtio-iommu node

hw/arm/virt: Fix devicetree warnings about the virtio-iommu node

The "PCI Bus Binding to: IEEE Std 1275-1994" defines the compatible
string for a PCIe bus or endpoint as "pci<vendorid>,<deviceid>" or
similar. Since the initial binding for PCI virtio-iommu didn't follow
this rule, it was modified to accept both strings and ensure backward
compatibility. Also, the unit-name for the node should be
"device,function".

Fix corresponding dt-validate and dtc warnings:

  pcie@10000000: virtio_iommu@16:compatible: ['virtio,pci-iommu'] does not contain items matching the given schema
  pcie@10000000: Unevaluated properties are not allowed (... 'virtio_iommu@16' were unexpected)
  From schema: linux/Documentation/devicetree/bindings/pci/host-generic-pci.yaml
  virtio_iommu@16: compatible: 'oneOf' conditional failed, one must be fixed:
        ['virtio,pci-iommu'] is too short
        'pci1af4,1057' was expected
  From schema: dtschema/schemas/pci/pci-bus.yaml

  Warning (pci_device_reg): /pcie@10000000/virtio_iommu@16: PCI unit address format error, expected "2,0"

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>