EXYNOS5250: Add support cpufreq for EXYNOS5250

This patch adds support cpufreq for EXYNOS5250 SoC. Basically,
the exynos-cpufreq.c is used commonly and exynos5250-cpufreq.c
is used for EXYNOS5250(two Cortex-A15 cores) SoC.

Signed-off-by: Jaecheol Lee <jc.lee@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Dave Jones <davej@redhat.com>