sunxi: only include alias for eMMC when mmc2 used

Some Allwinner SoCs (e.g. R329) doesn't have a MMC2 controller at all,
and on boards that we do not utilize MMC2, the alias for it is just
useless.

Only include the alias when we specify CONFIG_MMC_SUNXI_EXTRA_SLOT to 2.

Signed-off-by: Icenowy Zheng <icenowy@sipeed.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>