dt-bindings: usb: qcom,dwc3: Introduce interconnect properties for Qualcomm DWC3 driver

Add documentation for the interconnects and interconnect-names
properties for USB.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sandeep Maheswaram <sanm@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>