cpuidle: big.LITTLE: Add ARCH_EXYNOS entry in config

Add support to select generic big-little cpuidle driver for Samsung Exynos
series SoC's. This is required for Exynos big-llittle SoC's eg, Exynos5420.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>