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/musb-new/sunxi.c b/drivers/usb/musb-new/sunxi.c
index 6cf9826..d7170a3 100644
--- a/drivers/usb/musb-new/sunxi.c
+++ b/drivers/usb/musb-new/sunxi.c
@@ -535,7 +535,7 @@
 #ifdef CONFIG_USB_MUSB_HOST
 	.id		= UCLASS_USB,
 #else
-	.id		= UCLASS_USB_DEV_GENERIC,
+	.id		= UCLASS_USB_GADGET_GENERIC,
 #endif
 	.of_match	= sunxi_musb_ids,
 	.probe		= musb_usb_probe,