ARM: mvebu: the Armada XP Matrix board has 4 GB

Since the Armada XP Matrix board has 4 GB of RAM and not 2 GB, we
update the Device Tree to take into account the correct amount of
memory. As noted in the new comment, the last 256 MB of RAM are in
fact not usable, due to the overlap with the MBus Window address
range.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>