usb: dwc3: add glue layer dependencies

Glue layers for the DWC3 driver only make sense on specific platforms.
Add dependencies so that they are not built where they aren't needed.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>