dt-bindings: display: renesas,cmm: Add R-Car CMM documentation

Add device tree bindings documentation for the Renesas R-Car Display
Unit Color Management Module.

CMM is the image enhancement module available on each R-Car DU video
channel on R-Car Gen2 and Gen3 SoCs (V3H and V3M excluded).
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

Documentation/devicetree/bindings/display/renesas,cmm.yaml

+# SPDX-License-Identifier: GPL-2.0-only
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/renesas,cmm.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Renesas R-Car Color Management Module (CMM)
+
+maintainers:
+  - Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+  - Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
+  - Jacopo Mondi <jacopo+renesas@jmondi.org>
+
+description: |+
+  Renesas R-Car color management module connected to R-Car DU video channels.
+  It provides image enhancement functions such as 1-D look-up tables (LUT),
+  3-D look-up tables (CLU), 1D-histogram generation (HGO), and color
+  space conversion (CSC).
+
+properties:
+  compatible:
+    oneOf:
+      - items:
+        - enum:
+          - renesas,r8a7795-cmm
+          - renesas,r8a7796-cmm
+          - renesas,r8a77965-cmm
+          - renesas,r8a77990-cmm
+          - renesas,r8a77995-cmm
+        - const: renesas,rcar-gen3-cmm
+      - items:
+        - const: renesas,rcar-gen2-cmm
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  resets:
+    maxItems: 1
+
+  power-domains:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - resets
+  - power-domains
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/r8a7796-cpg-mssr.h>
+    #include <dt-bindings/power/r8a7796-sysc.h>
+
+    cmm0: cmm@fea40000 {
+         compatible = "renesas,r8a7796-cmm",
+                      "renesas,rcar-gen3-cmm";
+         reg = <0 0xfea40000 0 0x1000>;
+         power-domains = <&sysc R8A7796_PD_ALWAYS_ON>;
+         clocks = <&cpg CPG_MOD 711>;
+         resets = <&cpg 711>;
+    };