sunxi: bpi-m3: Enable eMMC

Add CONFIG_MMC_SUNXI_SLOT_EXTRA=2 to use eMMC on
BPI_M3 board.

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>