ARM: EXYNOS: convert cpuidle driver to be a platform driver

ARM EXYNOS cpuidle driver is currently not multiplatform support
ready (as it uses device_initcall() for initialization and doesn't
check on what platform it is running). Fix it by converting the
driver to be a platform driver and registering exynos_cpuidle
platform device from exynos[4,5]_dt_machine_init().

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>