sunxi: Imply USB_ETHER

Now that we can enable the usb_ether gadget, do it. This will be especially
useful for boards that don't have any ethernet controller, such as the ones
based on the A13 or A33.

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