schemas: pci: Drop 'compatible' as a required property for bridges

The pci-bus.yaml schema can apply to PCI to PCI bridges, but those may
not have a compatible string. Host bridges will require a compatible
anyways for matching their schema.

Reported-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>

schemas/pci/pci-bus.yaml

@@ -145,7 +145,6 @@ patternProperties:
       - reg
 
 required:
-  - compatible
   - device_type
   - ranges
   - "#address-cells"