uboot-imx

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>