PM / devfreq: exynos-ppmu: Use the regmap interface to handle the registers

This patch uses the regmap interface to read and write the registers for exynos
PPMU device instead of the legacy memory map functions.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>