ASoC: dt-bindings: Add Q6USB backend

Add a dt-binding to describe the definition of enabling the Q6 USB backend
device for audio offloading. The node carries information, which is passed
along to the QC USB SND class driver counterpart. These parameters will be
utilized during QMI stream enable requests.

Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Wesley Cheng <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/sound/qcom,q6usb.yaml

···
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/qcom,q6usb.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Qualcomm ASoC DPCM USB backend DAI
+
+maintainers:
+  - Wesley Cheng <[e-mail]>
+
+description:
+  The USB port is a supported AFE path on the Q6 DSP.  This ASoC DPCM
+  backend DAI will communicate the required settings to initialize the
+  XHCI host controller properly for enabling the offloaded audio stream.
+  Parameters defined under this node will carry settings, which will be
+  passed along during the QMI stream enable request and configuration of
+  the XHCI host controller.
+
+allOf:
+  - $ref: dai-common.yaml#
+
+properties:
+  compatible:
+    enum:
+      - qcom,q6usb
+
+  iommus:
+    maxItems: 1
+
+  "#sound-dai-cells":
+    const: 1
+
+  qcom,usb-audio-intr-idx:
+    description:
+      Desired XHCI interrupter number to use.  Depending on the audio DSP
+      on the platform, it will operate on a specific XHCI interrupter.
+    $ref: /schemas/types.yaml#/definitions/uint16
+    maximum: 8
+
+required:
+  - compatible
+  - "#sound-dai-cells"
+  - qcom,usb-audio-intr-idx
+
+additionalProperties: false
+
+examples:
+  - |
+    dais {
+      compatible = "qcom,q6usb";
+      #sound-dai-cells = <1>;
+      iommus = <&apps_smmu 0x180f 0x0>;
+      qcom,usb-audio-intr-idx = /bits/ 16 <2>;
+    };