PM / devfreq: exynos: make more PPMU code common

Move common PPMU code from exynos[45]_bus.c to exynos_ppmu.c.

There should be no functional changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>