The USB gadget framework does not support DM yet, so add this bit
to let DWC2 UDC probe from OF on platforms which support it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Lukasz Majewski <l.majewski@majess.pl>
Cc: Lukasz Majewski <l.majewski@samsung.com>