dt-bindings: display: renesas,du: Fix 'ports' reference

Fix the renesas,du binding 'ports' schema which is referencing the 'port'
schema instead of the 'ports' schema.

Fixes: 99d66127 ("dt-bindings: display: renesas,du: Convert binding to YAML")
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-renesas-soc@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20210623164308.2570164-1-robh@kernel.org

Documentation/devicetree/bindings/display/renesas,du.yaml

@@ -55,7 +55,7 @@ properties:
     maxItems: 1
 
   ports:
-    $ref: /schemas/graph.yaml#/properties/port
+    $ref: /schemas/graph.yaml#/properties/ports
     description: |
       The connections to the DU output video ports are modeled using the OF
       graph bindings specified in Documentation/devicetree/bindings/graph.txt.