dt-bindings: usb-connector: Use OF graph schema

Now that we have a graph schema, reference it from the usb-connector
schema.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20201102203656.220187-3-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/connector/usb-connector.yaml

@@ -143,22 +143,23 @@ properties:
       power dual role.
 
   ports:
-    description: OF graph bindings (specified in bindings/graph.txt) that model
-      any data bus to the connector unless the bus is between parent node and
-      the connector. Since a single connector can have multiple data buses every
-      bus has an assigned OF graph port number as described below.
-    type: object
+    $ref: /schemas/graph.yaml#/properties/ports
+    description: OF graph bindings modeling any data bus to the connector
+      unless the bus is between parent node and the connector. Since a single
+      connector can have multiple data buses every bus has an assigned OF graph
+      port number as described below.
+
     properties:
       port@0:
-        type: object
+        $ref: /schemas/graph.yaml#/properties/port
         description: High Speed (HS), present in all connectors.
 
       port@1:
-        type: object
+        $ref: /schemas/graph.yaml#/properties/port
         description: Super Speed (SS), present in SS capable connectors.
 
       port@2:
-        type: object
+        $ref: /schemas/graph.yaml#/properties/port
         description: Sideband Use (SBU), present in USB-C. This describes the
           alternate mode connection of which SBU is a part.