dt-bindings: arm: mediatek: mmsys: Add OF graph support for board path

Document OF graph on MMSYS/VDOSYS: this supports up to three DDP paths
per HW instance (so potentially up to six displays for multi-vdo SoCs).

The MMSYS or VDOSYS is always the first component in the DDP pipeline,
so it only supports an output port with multiple endpoints - where each
endpoint defines the starting point for one of the (currently three)
possible hardware paths.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Tested-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Tested-by: Michael Walle <mwalle@kernel.org> # on kontron-sbc-i1200
Link: https://lore.kernel.org/r/20241017103809.156056-3-angelogioacchino.delregno@collabora.com
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

Documentation/devicetree/bindings/arm/mediatek/mediatek,mmsys.yaml

@@ -93,6 +93,34 @@ properties:
   '#reset-cells':
     const: 1
 
+  port:
+    $ref: /schemas/graph.yaml#/properties/port
+    description:
+      Output port node. This port connects the MMSYS/VDOSYS output to
+      the first component of one display pipeline, for example one of
+      the available OVL or RDMA blocks.
+      Some MediaTek SoCs support multiple display outputs per MMSYS.
+    properties:
+      endpoint@0:
+        $ref: /schemas/graph.yaml#/properties/endpoint
+        description: Output to the primary display pipeline
+
+      endpoint@1:
+        $ref: /schemas/graph.yaml#/properties/endpoint
+        description: Output to the secondary display pipeline
+
+      endpoint@2:
+        $ref: /schemas/graph.yaml#/properties/endpoint
+        description: Output to the tertiary display pipeline
+
+    anyOf:
+      - required:
+          - endpoint@0
+      - required:
+          - endpoint@1
+      - required:
+          - endpoint@2
+
 required:
   - compatible
   - reg