ARM: EXYNOS: allow cpuidle driver usage on Exynos3250 SoC

Register cpuidle platform device on Exynos3250 SoC allowing EXYNOS
cpuidle driver usage on this SoC.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>