usb: dwc3: add ReST documentation

Document a few details about DWC3 in order to help people report bugs
and debug DWC3.
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

Documentation/driver-api/usb/index.rst

@@ -16,6 +16,7 @@ Linux USB API
    persist
    error-codes
    writing_usb_driver
+   dwc3
    writing_musb_glue_layer
 
 .. only::  subproject and html