ARM: EXYNOS: allow driver usage on Exynos4x12 SoCs

Register cpuidle platform device on Exynos4x12 SoCs allowing EXYNOS
cpuidle driver usage on these SoCs.

AFTR mode reduces power consumption on Trats2 board (Exynos4412 SoC
with secure firmware enabled) by ~12% when EXYNOS cpuidle driver is
enabled (in both cases the default exynos_defconfig config is used
and CPU1-3 are offlined).

Currently Exynos4412 SoC support is limited to Trats2 board.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>