ARM: exynos_defconfig: Enable CPU idle

Current Exynos CPU idle driver supports entering AFTR (Arm Off, Top
Running) mode on Exynos 4210 (coupled), Exynos 4x12 and Exynos 5250.
Enable it in default configuration to reduce energy consumption.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>