ARM: mvebu: Allow to power down L2 cache controller in idle mode

This commit adds a function which adjusts the PMSU configuration to
automatically power down the L2 and coherency fabric when we enter a
certain idle state.

This feature is part of the Power Management Service Unit of the
Armada 370 and Armada XP SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1397488214-20685-8-git-send-email-gregory.clement@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>