sunxi: A13-Olinuxino: Enable the USB OTG controller

The A13-Olinuxino has a mini-USB connector that can be used to power up
the board and as an OTG connector.

Since we have already some USB host-only ports right beside this one,
enable it in gadget mode

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>