dt-bindings: display: add Unisoc's drm master bindings

The Unisoc DRM master device is a virtual device needed to list all
DPU devices or other display interface nodes that comprise the
graphics subsystem

Unisoc's display pipeline have several components as below
description, multi display controllers and corresponding physical
interfaces.
For different display scenarios, dpu0 and dpu1 maybe binding to
different encoder.

E.g:
  dpu0 and dpu1 both binding to DSI for dual mipi-dsi display;
  dpu0 binding to DSI for primary display, and dpu1 binding to DP for
  external display;

Cc: Orson Zhai <orsonzhai@gmail.com>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Signed-off-by: Kevin Tang <kevin.tang@unisoc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211207142717.30296-2-kevin3.tang@gmail.com

Documentation/devicetree/bindings/display/sprd/sprd,display-subsystem.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/sprd/sprd,display-subsystem.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Unisoc DRM master device
+
+maintainers:
+  - Kevin Tang <kevin.tang@unisoc.com>
+
+description: |
+  The Unisoc DRM master device is a virtual device needed to list all
+  DPU devices or other display interface nodes that comprise the
+  graphics subsystem.
+
+  Unisoc's display pipeline have several components as below description,
+  multi display controllers and corresponding physical interfaces.
+  For different display scenarios, dpu0 and dpu1 maybe binding to different
+  encoder.
+
+  E.g:
+  dpu0 and dpu1 both binding to DSI for dual mipi-dsi display;
+  dpu0 binding to DSI for primary display, and dpu1 binding to DP for external display;
+
+          +-----------------------------------------+
+          |                                         |
+          |                            +---------+  |
+  +----+  |   +----+    +---------+    |DPHY/CPHY|  |   +------+
+  |    +----->+dpu0+--->+MIPI|DSI +--->+Combo    +----->+Panel0|
+  |AXI |  |   +----+    +---------+    +---------+  |   +------+
+  |    |  |                  ^                      |
+  |    |  |                  |                      |
+  |    |  |      +-----------+                      |
+  |    |  |      |                                  |
+  |APB |  |   +--+-+    +-----------+    +---+      |   +------+
+  |    +----->+dpu1+--->+DisplayPort+--->+PHY+--------->+Panel1|
+  |    |  |   +----+    +-----------+    +---+      |   +------+
+  +----+  |                                         |
+          +-----------------------------------------+
+
+properties:
+  compatible:
+    const: sprd,display-subsystem
+
+  ports:
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+    description:
+      Should contain a list of phandles pointing to display interface port
+      of DPU devices.
+
+required:
+  - compatible
+  - ports
+
+additionalProperties: false
+
+examples:
+  - |
+    display-subsystem {
+        compatible = "sprd,display-subsystem";
+        ports = <&dpu_out>;
+    };
+