dt-bindings: dp-connector: add binding for DisplayPort connector

Add binding for DisplayPort connector. A few notes:

* Similar to hdmi-connector, it has hpd-gpios as an optional property,
  as the HPD could also be handled by, e.g., the DP bridge.

* dp-pwr-supply, which provides 3.3V on DP_PWR pin, is optional, as it
  is not strictly required: standard DP cables do not even have the pin
  connected.

* Connector type. Full size and mini connectors are identical except for
  the connector size and form, so I believe there is no functional need
  for this property. But similar to 'label' property, it might be used
  to present information about the connector to the userspace.

* No eDP. There's really no "eDP connector", as it's always a custom
  made connection between the DP and the DP panel, although the eDP spec
  does offer a few suggested pin setups. So possibly there is no need for
  edp-connector binding, but even if there is, I don't want to guess what
  it could look like, and could it be part of the dp-connector binding.

* No DP++. I'm not familiar with DP++. DP++ might need an i2c bus added
  to the bindings.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130112919.241054-2-tomi.valkeinen@ti.com

Documentation/devicetree/bindings/display/connector/dp-connector.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/connector/dp-connector.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DisplayPort Connector
+
+maintainers:
+  - Tomi Valkeinen <tomi.valkeinen@ti.com>
+
+properties:
+  compatible:
+    const: dp-connector
+
+  label: true
+
+  type:
+    enum:
+      - full-size
+      - mini
+
+  hpd-gpios:
+    description: A GPIO line connected to HPD
+    maxItems: 1
+
+  dp-pwr-supply:
+    description: Power supply for the DP_PWR pin
+    maxItems: 1
+
+  port:
+    $ref: /schemas/graph.yaml#/properties/port
+    description: Connection to controller providing DP signals
+
+required:
+  - compatible
+  - type
+  - port
+
+additionalProperties: false
+
+examples:
+  - |
+    connector {
+        compatible = "dp-connector";
+        label = "dp0";
+        type = "full-size";
+
+        port {
+            dp_connector_in: endpoint {
+                remote-endpoint = <&dp_out>;
+            };
+        };
+    };
+
+...