ARM: exynos_defconfig: Enable cpufreq-dt driver

With the latest patches the cpufreq-dt can be used on multiple
Exynos SoCs: 3250, 4210, 4212, 4412 and 5250.

Enable it along with default ondemand governor to conserve the energy,
reduce temperature while maintaining acceptable performance.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>