ARM: Exynos: use generic cpufreq driver for Exynos5420

ARM: Exynos: use generic cpufreq driver for Exynos5420

The new CPU clock type allows the use of generic arm_big_little_dt
cpufreq driver for Exynos5420.

Changes by Bartlomiej:
- split Exynos5420 support from the original patch
- disable cpufreq if big.LITTLE switcher support is enabled

Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>