dt-bindings: usb: keystone-dwc3: Validate DWC3 sub-node

TI Keystone DWC3 compatible DT node is supposed to have a DWC USB3
compatible sub-node to describe a fully functioning USB interface.
Since DWC USB3 has now got a DT schema describing its DT node, let's make
sure the TI Keystone DWC3 sub-node passes validation against it.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201210090944.16283-18-Sergey.Semin@baikalelectronics.ruSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/ti,keystone-dwc3.yaml

@@ -64,9 +64,7 @@ properties:
 
 patternProperties:
   "usb@[a-f0-9]+$":
-    type: object
-    description: This is the node representing the DWC3 controller instance
-      Documentation/devicetree/bindings/usb/dwc3.txt
+    $ref: snps,dwc3.yaml#
 
 required:
   - compatible