docs: dt: qcom,dwc3.txt: fix cross-reference for a converted file

The qcom-qusb2-phy.txt file was converted and renamed to yaml.
Update cross-reference accordingly.

Fixes: 8ce65d8d ("dt-bindings: phy: qcom,qusb2: Convert QUSB2 phy bindings to yaml")
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a055c564f2a79aa748064329d938db8b3c8edd58.1586881715.git.mchehab+huawei@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/devicetree/bindings/usb/qcom,dwc3.txt

@@ -53,7 +53,7 @@ the node is not important. The content of the node is defined in dwc3.txt.
 
 Phy documentation is provided in the following places:
 Documentation/devicetree/bindings/phy/qcom-qmp-phy.txt    - USB3 QMP PHY
-Documentation/devicetree/bindings/phy/qcom-qusb2-phy.txt - USB2 QUSB2 PHY
+Documentation/devicetree/bindings/phy/qcom,qusb2-phy.yaml - USB2 QUSB2 PHY
 
 Example device nodes: