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/gadget/ether.c b/drivers/usb/gadget/ether.c
index 5a9ffd7..3b3d9af 100644
--- a/drivers/usb/gadget/ether.c
+++ b/drivers/usb/gadget/ether.c
@@ -2671,7 +2671,7 @@
 	struct udevice *usb_dev;
 	int ret;
 
-	ret = uclass_first_device(UCLASS_USB_DEV_GENERIC, &usb_dev);
+	ret = uclass_first_device(UCLASS_USB_GADGET_GENERIC, &usb_dev);
 	if (!usb_dev || ret) {
 		pr_err("No USB device found\n");
 		return ret;