PM / devfreq: Move exynos4 devfreq driver into a new sub-directory

In anticipation of the new exynos5 devfreq and ppmu driver, create
an exynos sub-directory. Move the existing exynos4 devfreq driver
into the same.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Jonghwan Choi <jhbird.choi@samsung.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>