ARM: Odroid XU3: Fix autoboot.cmd to use ${mmcbootdev} instead of hardcoded 0

This commit adjusts the autoboot.cmd file to use ${mmcbootdev} instead of
hardcoded value 0.

This is necessary to allow booting this board from the SD card.

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