sunxi: Enable USB gadget support for Sinlinx SinA33

Sinlinx SinA33 has a USB OTG port, but VBUS is controlled manually from
a jumper pad.

Enable OTG in gadget mode, as well as the download gadget and related
functions.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>