cpuidle: Enable coupled cpuidle support on Exynos3250 platform

All the needed code has been already merged to mach-exynos core in
commit af997114

 ("ARM: EXYNOS: add coupled cpuidle support for
Exynos3250"), so enable support for coupled variant also for Exynos3250
SoCs.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>