dm: usb: create a new UCLASS ID for USB gadget devices

UCLASS_USB_DEV_GENERIC was meant for USB devices connected to host
controllers, not gadget devices.
Adding a new UCLASS for gadget devices alone.

Also move the generic DM code for USB gadgets in a separate file for
clarity.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/drivers/usb/dwc3/dwc3-generic.c b/drivers/usb/dwc3/dwc3-generic.c
index 681b5c7..7375660 100644
--- a/drivers/usb/dwc3/dwc3-generic.c
+++ b/drivers/usb/dwc3/dwc3-generic.c
@@ -72,7 +72,7 @@
 
 U_BOOT_DRIVER(dwc3_generic_peripheral) = {
 	.name	= "dwc3-generic-peripheral",
-	.id	= UCLASS_USB_DEV_GENERIC,
+	.id	= UCLASS_USB_GADGET_GENERIC,
 	.ofdata_to_platdata = dwc3_generic_peripheral_ofdata_to_platdata,
 	.probe = dwc3_generic_peripheral_probe,
 	.remove = dwc3_generic_peripheral_remove,