ARM: S5PV310: Add FOUT APLL get rate function

FOUT APLL clock is used as a source of ARM core clock. So we need that the
clock source can be changed dynamically by using CPUFREQ driver. This patch
can give correct frequency when calling clk_get_rate() function.

Signed-off-by: Jaecheol Lee <jc.lee@samsung.com>
Signed-off-by: Sangwook Ju <sw.ju@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>