usb: gadget: introduce gadget_is_dwc3()

... to check whether we're running on DesignWare
USB3 DRD Controller.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
[idos@codeaurora.org: Conflicts: gadget_chips.h]
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>