arm: mvebu: Add initial support for power managmement service unit

The Armada 370 and Armada XP SOCs have a power management service unit
which is responsible for powering down and waking up CPUs and other
SOC units. This patch adds support for this unit.

Signed-off-by: Yehuda Yitschak <yehuday@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>