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>