PM / devfreq: exynos4: use common PPMU code

This patch converts exynos4_bus driver to use common PPMU code
(exynos_ppmu.c) instead of individual functions related to PPC
because PPMU is integrated module with both PPC and Bus event
generator.  When using PPMU to get bus performance read/write
event exynos4_bus driver deson't need to consider memory type.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
[bzolnier: splitted out changes from the bigger patch]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>