ARM: mvebu: Add the PMSU related part of the cpu idle functions

The cpu idle support will need to access to Power Management Service
Unit. This commit adds the architecture related functions that will be
used in the idle path of the cpuidle driver.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Link: https://lkml.kernel.org/r/1397488214-20685-9-git-send-email-gregory.clement@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>