PM / devfreq: update the name of EXYNOS clock register

According to replacing the name of EXYNOS clock registers,
this patch updates exynos4_bus.c file where it is used.
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>