ARM: exynos5: Try to boot on mmc2 before mmc0/1

As stated in commit a61a4a1d with DM_MMC,
exynos boards now enumarates external SD/MMC slot as mmc2, instead of mmc1
with legacy mode. Moving mmc2 before mmc1/0 restore the previous behavior
of trying external SD/MMC before internal slot.

Signed-off-by: Guillaume GARDET <guillaume.gardet@arm.com>

Cc: Lukasz Majewski <lukma@denx.de>
Cc: Anand Moon <linux.amoon@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>