sunxi: imply USB_GADGET

A good number of our boards have USB_GADGET enabled. Imply it so that all
the boards can benefit from it, and remove some boilerplate from our
defconfigs.

Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>