ARM: mvebu: add CPU config registers in the Armada 370/XP Device Tree

This commit adds the description of the CPU config registers in the
Armada 370 and Armada XP Device Tree. Since the registers are in fact
different between the two SoCs, a different compatible string is used.

Note that the Armada 370 node is currently unused, but it is
nonetheless added for consistency with the addition on the Armada XP
side.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>