ARM: Odroid XU3: Adjust BOOT_TARGET_DEVICES to allow booting from SD card (mmc2)

This change is necessary to allow booting the Odroid XU3 from SD card
after enabling the DM_MMC support.

After this change the SD card mmc IP block is correctly enumerated as mmc2
(and not as mmc1 as in the legacy code).

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