PM / devfreq: exynos5: introduce struct busfreq_ppmu_data

This is a preparation for making more PPMU code common for
EXYNOS devfreq drivers.

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>