dt-bindings: Remove plain text OF graph binding

A schema for the OF graph binding has been added to the dt-schema repo
based on graph.txt contents. Let's replace graph.txt now duplicated
contents with a reference to the schema.

For users of the graph binding, they should reference to the graph
schema from either 'ports' or 'port' property:

properties:
  ports:
    $ref: /schemas/graph.yaml#/properties/ports

    properties:
      port@0:
        $ref: /schemas/graph.yaml#/properties/port
        description: What data this port has

      ...

Or:

properties:
  port:
    description: What data this port has
    $ref: /schemas/graph.yaml#/properties/port

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sameer Pujar <spujar@nvidia.com>
Link: https://lore.kernel.org/r/20210112154631.406250-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>