Exynos5: Add support for USB download boot mode

Exynos5250 supports secondary USB device boot mode. If the iROM fails
to download u-boot from the primary boot device (such as SD or eMMC),
it will try to retrieve from the secondary boot device (such as USB).

Signed-off-by: Naveen Krishna Ch <ch.naveen@samsung.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>