ARM: mvebu: Add standby support

Until now only one Armada XP and one Armada 388 based board supported
suspend to ram. However, most of the recent mvebu SoCs can support the
standby mode. Unlike for the suspend to ram, nothing special has to be
done for these SoCs. This patch allows the system to use the standby
mode on Armada 370, 38x, 39x and XP SoCs. There are issues with the
Armada 375, and the support might be added (if possible) in a future
patch.

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