dt-bindings: virtio: pci-iommu: Add ref to pci-device.yaml

The virtio pci-iommu is a PCI device, so it should have a reference to
the pci-device.yaml schema. The pci-device.yaml schema defines the 'reg'
format as a schema, so the text description for 'reg' can be dropped.

Reviewed-by: Jean-Philippe Brucker <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/virtio/pci-iommu.yaml

···
   virtio-iommu node doesn't have an "iommus" property, and is omitted from
   the iommu-map property of the root complex.
 
+allOf:
+  - $ref: /schemas/pci/pci-device.yaml#
+
 properties:
   # If compatible is present, it should contain the vendor and device ID
   # according to the PCI Bus Binding specification. Since PCI provides
···
           - const: pci1af4,1057
 
   reg:
-    description: |
-      PCI address of the IOMMU. As defined in the PCI Bus Binding
-      reference, the reg property is a five-cell address encoded as (phys.hi
-      phys.mid phys.lo size.hi size.lo). phys.hi should contain the device's
-      BDF as 0b00000000 bbbbbbbb dddddfff 00000000. The other cells should be
-      zero. See Documentation/devicetree/bindings/pci/pci.txt
+    maxItems: 1
 
   '#iommu-cells':
     const: 1