OMAPDSS: Correct video ports description file path in DT binding doc

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The DSS Core and the encoders have video port outputs. The structure of the

video ports is described in Documentation/devicetree/bindings/video/video-

ports.txt, and the properties for the ports and endpoints for each encoder are

video ports is described in Documentation/devicetree/bindings/graph.txt,

and the properties for the ports and endpoints for each encoder are

described in the SoC's DSS binding documentation.

The video ports are used to describe the connections to external hardware, like