ARM: dts: sun9i: Enable mmc2 on A80 Optimus Board

The A80 Optimus Board has a 16GB eMMC connected to mmc2, with 8 bit
wide data bus.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>