ASoC: dt-bindings: Convert SPDIF Transmitter binding to a schema

The SPDIF Transmitter binding is used by Linux with a matching Device
Tree binding.

Now that we have the DT validation in place, let's convert the device
tree bindings for that driver over to a YAML schema.

Cc: alsa-devel@alsa-project.org
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMaxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210901091852.479202-4-maxime@cerno.techSigned-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/linux,spdif-dit.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/linux,spdif-dit.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Dummy SPDIF Transmitter Device Tree Bindings
+
+maintainers:
+  - Mark Brown <broonie@kernel.org>
+
+properties:
+  compatible:
+    const: linux,spdif-dit
+
+  "#sound-dai-cells":
+    const: 0
+
+required:
+  - "#sound-dai-cells"
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    spdif-out {
+        #sound-dai-cells = <0>;
+        compatible = "linux,spdif-dit";
+    };
+
+...

Documentation/devicetree/bindings/sound/spdif-transmitter.txt

-Device-Tree bindings for dummy spdif transmitter
-
-Required properties:
-	- compatible: should be "linux,spdif-dit".
-
-Example node:
-
-	codec: spdif-transmitter {
-		compatible = "linux,spdif-dit";
-	};