configs: sun50i: Enable eMMC on a64-olinuxino

a64-olinuxino has 8GiB eMMC, enable it.

Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>

configs/a64-olinuxino_defconfig

@@ -2,6 +2,7 @@ CONFIG_ARM=y
 CONFIG_ARCH_SUNXI=y
 CONFIG_MACH_SUN50I=y
 CONFIG_RESERVE_ALLWINNER_BOOT0_HEADER=y
+CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 CONFIG_DEFAULT_DEVICE_TREE="sun50i-a64-olinuxino"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL=y