cpufreq: exynos: Split exynos_target function into two functions

Split exynos_target function into exynos_target & exynos_cpufreq_scale.
The exynos_cpufreq_scale changes the voltage & frequency.

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>