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>