ARM: mvebu: convert Marvell Armada XP GP to PBL_MULTI_IMAGES
This converts Marvell Armada XP based Marvell Armada XP GP
to PBL_MULTI_IMAGES. A DT overlay is added to keep possible
barebox-specific changes separated and added to lowlevel
board init.
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>