ARM: ux500: cpuidle: Move ux500 cpuidle driver to drivers/cpuidle

There is no more dependency with arch/arm headers, so we can safely move the
driver to the drivers/cpuidle directory.

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