cpufreq: exynos: remove Exynos4210 specific cpufreq driver support

Exynos4210 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.

Changes by Bartlomiej:
- dropped Exynos5250 support removal for now
- updated exynos-cpufreq.[c,h]

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