arm: socfpga: Allow DWC2 UDC probing from OF

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>