ASoC: add simple-audio-mux binding

Add devicetree documentation for simple audio multiplexers
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201205001508.346439-1-alexandre.belloni@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/simple-audio-mux.yaml

+# SPDX-License-Identifier: (GPL-2.0+ OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/simple-audio-mux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Simple Audio Multiplexer
+
+maintainers:
+  - Alexandre Belloni <aleandre.belloni@bootlin.com>
+
+description: |
+  Simple audio multiplexers are driven using gpios, allowing to select which of
+  their input line is connected to the output line.
+
+properties:
+  compatible:
+    const: simple-audio-mux
+
+  mux-gpios:
+    description: |
+      GPIOs used to select the input line.
+
+  sound-name-prefix:
+    $ref: /schemas/types.yaml#/definitions/string
+    description:
+      Used as prefix for sink/source names of the component. Must be a
+      unique string among multiple instances of the same component.
+
+required:
+  - compatible
+  - mux-gpios
+
+additionalProperties: false
+
+examples:
+  - |
+    mux {
+        compatible = "simple-audio-mux";
+        mux-gpios = <&gpio 3 0>;
+    };