vexpress64: Add MMC card to the BOOT_TARGET_DEVICES of FVP

Add MMC disk to FVP's BOOT_TARGET_DEVICES. This allows the user to boot
from MMC devices.

Signed-off-by: Wei Chen <wei.chen@arm.com>
Signed-off-by: Qi Feng <qi.feng@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>