ARM: ux500: add the cpuidle driver for WFI and ARM retention

This patch adds the cpuidle driver for the ux500 SoC.
The boards saves 12mA with these states. It is based on the latest
cpuidle consolidation from Robert Lee.

The cpu can go to retention only if the other core is in WFI.

If the other cpu is in WFI and we decoupled the gic from the cores,
then we have the guarantee, it won't be wake up.

It is up to the prcmu firmware to recouple the gic automatically
after the power state mode is selected.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>