dt-bindings: PCI: brcmstb: compatible is required

The compatible property is required, make sure the binding documents it
as such.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211202223609.1171452-1-f.fainelli@gmail.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/brcm,stb-pcie.yaml

@@ -76,6 +76,7 @@ properties:
       maxItems: 3
 
 required:
+  - compatible
   - reg
   - ranges
   - dma-ranges