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 <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: [e-mail]
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
       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