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

Documentation/devicetree/bindings/graph.txt

-Common bindings for device graphs
-
-General concept
----------------
-
-The hierarchical organisation of the device tree is well suited to describe
-control flow to devices, but there can be more complex connections between
-devices that work together to form a logical compound device, following an
-arbitrarily complex graph.
-There already is a simple directed graph between devices tree nodes using
-phandle properties pointing to other nodes to describe connections that
-can not be inferred from device tree parent-child relationships. The device
-tree graph bindings described herein abstract more complex devices that can
-have multiple specifiable ports, each of which can be linked to one or more
-ports of other devices.
-
-These common bindings do not contain any information about the direction or
-type of the connections, they just map their existence. Specific properties
-may be described by specialized bindings depending on the type of connection.
-
-To see how this binding applies to video pipelines, for example, see
-Documentation/devicetree/bindings/media/video-interfaces.txt.
-Here the ports describe data interfaces, and the links between them are
-the connecting data buses. A single port with multiple connections can
-correspond to multiple devices being connected to the same physical bus.
-
-Organisation of ports and endpoints
------------------------------------
-
-Ports are described by child 'port' nodes contained in the device node.
-Each port node contains an 'endpoint' subnode for each remote device port
-connected to this port. If a single port is connected to more than one
-remote device, an 'endpoint' child node must be provided for each link.
-If more than one port is present in a device node or there is more than one
-endpoint at a port, or a port node needs to be associated with a selected
-hardware interface, a common scheme using '#address-cells', '#size-cells'
-and 'reg' properties is used to number the nodes.
-
-device {
-        ...
-        #address-cells = <1>;
-        #size-cells = <0>;
-
-        port@0 {
-	        #address-cells = <1>;
-	        #size-cells = <0>;
-		reg = <0>;
-
-                endpoint@0 {
-			reg = <0>;
-			...
-		};
-                endpoint@1 {
-			reg = <1>;
-			...
-		};
-        };
-
-        port@1 {
-		reg = <1>;
-
-		endpoint { ... };
-	};
-};
-
-All 'port' nodes can be grouped under an optional 'ports' node, which
-allows to specify #address-cells, #size-cells properties for the 'port'
-nodes independently from any other child device nodes a device might
-have.
-
-device {
-        ...
-        ports {
-                #address-cells = <1>;
-                #size-cells = <0>;
-
-                port@0 {
-                        ...
-                        endpoint@0 { ... };
-                        endpoint@1 { ... };
-                };
-
-                port@1 { ... };
-        };
-};
-
-Links between endpoints
------------------------
-
-Each endpoint should contain a 'remote-endpoint' phandle property that points
-to the corresponding endpoint in the port of the remote device. In turn, the
-remote endpoint should contain a 'remote-endpoint' property. If it has one, it
-must not point to anything other than the local endpoint. Two endpoints with
-their 'remote-endpoint' phandles pointing at each other form a link between the
-containing ports.
-
-device-1 {
-        port {
-                device_1_output: endpoint {
-                        remote-endpoint = <&device_2_input>;
-                };
-        };
-};
-
-device-2 {
-        port {
-                device_2_input: endpoint {
-                        remote-endpoint = <&device_1_output>;
-                };
-        };
-};
-
-Required properties
--------------------
-
-If there is more than one 'port' or more than one 'endpoint' node or 'reg'
-property present in the port and/or endpoint nodes then the following
-properties are required in a relevant parent node:
-
- - #address-cells : number of cells required to define port/endpoint
-                    identifier, should be 1.
- - #size-cells    : should be zero.
-
-Optional endpoint properties
-----------------------------
-
-- remote-endpoint: phandle to an 'endpoint' subnode of a remote device node.
-
+This file has moved to graph.yaml in dt-schema repo