cpufreq: exynos: remove Exynos5250 specific cpufreq driver support

cpufreq: exynos: remove Exynos5250 specific cpufreq driver support

Exynos5250 based platforms have switched over to use generic
cpufreq driver for cpufreq functionality. So the Exynos
specific cpufreq support for these platforms can be removed.

The exynos-cpufreq driver itself is also removed as it is no
longer used/needed after Exynos5250 support removal.

Based on the earlier work by Thomas Abraham.

Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Cc: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>