ASoC: dt-bindings: mediatek,mt2701-hdmi-audio: add MT2701 HDMI audio

Describe the sound card node that routes the MT2701/MT7623N AFE
HDMI playback path to the on-chip HDMI transmitter. This is
separate from the AFE platform binding (mediatek,mt2701-audio)
because it represents board-level audio routing between the AFE
and the HDMI codec, not an additional IP block. MT7623N boards
carry the same IP and use the mt7623n- compatible as a fallback
to mt2701-.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/b18cd550bcfab8c5a8a765d68a9ab5bae3f1c4e7.1776998727.git.daniel@makrotopia.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mediatek,mt2701-hdmi-audio.yaml

@@ -0,0 +1,48 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/mediatek,mt2701-hdmi-audio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MediaTek MT2701/MT7623N HDMI audio output
+
+maintainers:
+  - Daniel Golle <daniel@makrotopia.org>
+
+description:
+  Sound card routing the MT2701/MT7623N Audio Front End HDMI
+  playback path to the on-chip HDMI transmitter. The AFE
+  provides the DMA memif and internal I2S engine; the HDMI
+  transmitter acts as the audio codec on the serialised link.
+
+properties:
+  compatible:
+    oneOf:
+      - const: mediatek,mt2701-hdmi-audio
+      - items:
+          - const: mediatek,mt7623n-hdmi-audio
+          - const: mediatek,mt2701-hdmi-audio
+
+  mediatek,platform:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description: Phandle of the MT2701/MT7623N AFE platform node.
+
+  mediatek,audio-codec:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description: Phandle of the HDMI transmitter acting as audio codec.
+
+required:
+  - compatible
+  - mediatek,platform
+  - mediatek,audio-codec
+
+additionalProperties: false
+
+examples:
+  - |
+    sound-hdmi {
+        compatible = "mediatek,mt7623n-hdmi-audio",
+                     "mediatek,mt2701-hdmi-audio";
+        mediatek,platform = <&afe>;
+        mediatek,audio-codec = <&hdmi0>;
+    };