dt-bindings: display: mediatek, dpi: Convert to use graph schema

Update the mediatek,dpi binding to use the graph schema. Missed
this one from the mass conversion since it's not part of drm-misc.

Cc: Chun-Kuang Hu <[email protected]>
Cc: Philipp Zabel <[email protected]>
Cc: Matthias Brugger <[email protected]>
Cc: CK Hu <[email protected]>
Cc: Jitao shi <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Chun-Kuang Hu <[email protected]>

Author: robherring

Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.yaml

@@ -50,15 +50,10 @@ properties:
       - const: sleep
 
   port:
-    type: object
+    $ref: /schemas/graph.yaml#/properties/port
     description:
-      Output port node with endpoint definitions as described in
-      Documentation/devicetree/bindings/graph.txt. This port should be connected
-      to the input port of an attached HDMI or LVDS encoder chip.
-
-    properties:
-      endpoint:
-        type: object
+      Output port node. This port should be connected to the input port of an
+      attached HDMI or LVDS encoder chip.
 
 required:
   - compatible