EXYNOS4X12: Add support cpufreq for EXYNOS4X12

This patch adds support cpufreq for EXYNOS4X12 SoCs. Basically,
the exynos-cpufreq.c is used commonly and exynos4x12-cpufreq.c
is used for EXYNOS4212(two Cortex-A9 cores) and EXYNOS4412(four
Cortex-A9 cores) SoCs.

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>