sunxi: provide default USB gadget setup

All the Allwinner boards use the same manufacturer, VID and PID for the
gadgets. Make them the defaults to 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>