USB: gadget: fix imx_udc entry in Kconfig

Move USB_GADGET_IMX to the right section of Kconfig as this
controller is available only as integrated on i.MX CPU.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>