sunxi: Enable eMMC on Cubietruck Plus

Set CONFIG_MMC_SUNXI_SLOT_EXTRA=2 to enable the eMMC controller to
access eMMC on the board.

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