mfd: Decouple/recouple gic from the ux500 PRCMU

This patch allows to decouple and recouple the gic from the PRCMU.
This is needed to put the A9 core in retention mode with the cpuidle
driver.

It is based on top of the "DB8500 PRCMU update" patchset.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Rickard Andersson <rickard.andersson@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>