ARM: mvebu: add SDRAM controller description for Armada XP

The suspend/resume sequence on Armada XP needs to modify a number of
registers in the SDRAM controller. Therefore, this commit updates the
Armada XP Device Tree description to include the SDRAM controller
Device Tree node.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1416585613-2113-17-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>