rockchip: rk3328: board: add support of dwc2 gadget

Probe dwc2 udc in the function of board_usb_start to enable
usb gadget function.

Signed-off-by: Meng Dongyang <daniel.meng@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>