usb: dwc3: Fix USB gadget initialization sequence

This change fixes below issues:
1. registering UDC driver with udc core with dwc3 probe itself, so USB
composition script can find UDC device while trying to select UDC.
2. creation of required USB endpoints related debugfs entries useful for
debugging.
3. USB high speed only functionality fallback mechanism when USB super
speed phy is not initialized.

Change-Id: I893e6221ceb6af448695f5399e857be0c25f41b1
Signed-off-by: Mayank Rana <mrana@codeaurora.org>