PM / devfreq: move definitions for exynos4_bus into drivers/devfreq

We don't need to keep the definitions for exynos4_bus into
mach-exynos/ so this moves them into drviers/devfreq with
adding header file.

Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>