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 cpufreq-dt driver
for Exynos5420.

Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Javier Martinez Canillas <javier@osg.samsung.com>
Cc: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>