ASoC: dt-bindings: qcom,q6afe: Document q6usb subnode

Document the subnode for Q6USB, used for USB audio offloading.

Cc: Wesley Cheng <quic_wcheng@quicinc.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Link: https://patch.msgid.link/20250723-fp4-usb-audio-offload-v3-1-6be84ed4fc39@fairphone.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/qcom,q6afe.yaml

@@ -29,6 +29,12 @@ properties:
     unevaluatedProperties: false
     description: Qualcomm DSP audio ports
 
+  usbd:
+    type: object
+    $ref: /schemas/sound/qcom,q6usb.yaml#
+    unevaluatedProperties: false
+    description: Qualcomm DSP USB audio ports
+
 required:
   - compatible
   - dais
@@ -64,5 +70,12 @@ examples:
                     qcom,sd-lines = <0 1 2 3>;
                 };
             };
+
+            usbd {
+                compatible = "qcom,q6usb";
+                #sound-dai-cells = <1>;
+                iommus = <&apps_smmu 0x180f 0x0>;
+                qcom,usb-audio-intr-idx = /bits/ 16 <2>;
+            };
         };
     };