dt-bindings: reserved-memory: Support framebuffer reserved memory

Document the "framebuffer" compatible string for reserved memory nodes
to annotate reserved memory regions used for framebuffer carveouts.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-4-thierry.reding@gmail.com

Documentation/devicetree/bindings/reserved-memory/framebuffer.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/reserved-memory/framebuffer.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: /reserved-memory framebuffer node bindings
+
+maintainers:
+  - devicetree-spec@vger.kernel.org
+
+allOf:
+  - $ref: reserved-memory.yaml
+
+properties:
+  compatible:
+    const: framebuffer
+    description: >
+      This indicates a region of memory meant to be used as a framebuffer for
+      a set of display devices. It can be used by an operating system to keep
+      the framebuffer from being overwritten and use it as the backing memory
+      for a display device (such as simple-framebuffer).
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    / {
+        compatible = "foo";
+        model = "foo";
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        chosen {
+            framebuffer {
+                compatible = "simple-framebuffer";
+                memory-region = <&fb>;
+            };
+        };
+
+        reserved-memory {
+            #address-cells = <1>;
+            #size-cells = <1>;
+            ranges;
+
+            fb: framebuffer@80000000 {
+                compatible = "framebuffer";
+                reg = <0x80000000 0x007e9000>;
+            };
+        };
+    };
+...