dt-bindings: usb: Update documentation for Qualcomm DWC3 driver

Existing documentation has lot of incorrect information as it
was originally added for a driver that no longer exists.

Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>