ASoC: dt-bindings: Add WM8978 Binding

Even though we had the wm8978 driver for some time and a number of
boards using it already, we never had a binding for it. Let's add it
based on what the driver expects and the boards are providing.

Cc: alsa-devel@alsa-project.org
Cc: devicetree@vger.kernel.org
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: patches@opensource.cirrus.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210901091852.479202-2-maxime@cerno.techSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/wlf,wm8978.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/wlf,wm8978.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Wolfson WM8978 Codec Device Tree Bindings
+
+maintainers:
+  - patches@opensource.cirrus.com
+
+properties:
+  '#sound-dai-cells':
+    const: 0
+
+  compatible:
+    const: wlf,wm8978
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency:
+    maximum: 526000
+
+required:
+  - '#sound-dai-cells'
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        codec@0 {
+            #sound-dai-cells = <0>;
+            compatible = "wlf,wm8978";
+            reg = <0>;
+            spi-max-frequency = <500000>;
+        };
+    };
+
+  - |
+    i2c {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        codec@0 {
+            #sound-dai-cells = <0>;
+            compatible = "wlf,wm8978";
+            reg = <0>;
+        };
+    };
+
+...