dm: usb: gadget: Fix boot breakage on sunxi platforms

Fixes commit 01311624

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

The UCLASS_DRIVER for id UCLASS_USB_GADGET_GENERIC needs to be declared
even for platforms that do not enable DM_USB_GADGET. Otherwise the driver
for their usb peripheral controller fails to bind.

Reported-by: Priit Laes <plaes@plaes.org>
Reported-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Tested-by: Priit Laes <plaes@plaes.org>
Acked-by: Jagan Teki <jagan@openedev.com>