ARM: exynos_defconfig: Enable Control Groups support

systemd needs control groups support to be enabled in the
kernel so let's enable it by default since is quite likely
that a user-space with systemd will be used.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>