mmc: sunxi: Enable 8bits bus width for sun8i

The sun8i SoCs also have a 8 bits capable MMC2 controller. Enable the
support for those too.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>