dt-bindings: usb: update snps,dwc3.yaml references

Changeset 389d7765

 ("dt-bindings: usb: Convert DWC USB3 bindings to DT schema")
renamed: Documentation/devicetree/bindings/usb/dwc3.txt
to: Documentation/devicetree/bindings/usb/snps,dwc3.yaml.

Update its cross-references accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/97704f110f0282fb47eb85dea430cc94cfd93a4b.1610605373.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>