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

The doc refers to Documentation/devicetree/bindings/video/video-ports.txt
which does not exist. The documentation seems to be outdated and wants to
refer to Documentation/devicetree/bindings/graph.txt instead.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>