ARM: multi_v7_defconfig: Enable CPU idle for exynos SoCs

Current Exynos CPU idle driver supports entering AFTR (Arm Off, Top
Running) mode on Exynos3250, Exynos4210 (coupled), Exynos4x12 and
Exynos5250. Enable it in default configuration to reduce energy
consumption.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>