dt-bindings: usb: mediatek,mtu3: Use graph schema

Users of the DT graph binding now should reference it for their 'port'
schemas, so add it for Mediatek MTU3 binding.

Cc: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210327191448.410795-1-robh@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/mediatek,mtu3.yaml

@@ -127,7 +127,7 @@ properties:
       Any connector to the data bus of this controller should be modelled
       using the OF graph bindings specified, if the "usb-role-switch"
       property is used. See graph.txt
-    type: object
+    $ref: /schemas/graph.yaml#/properties/port
 
   enable-manual-drd:
     $ref: /schemas/types.yaml#/definitions/flag