ARM: EXYNOS: add exynos3250 PMU support

This patch prepares the PMU code for the future:
- suspend/resume (S2R) support
- cpuidle AFTR/W-AFTR modes support
on Exynos3250.

Cc: Vikas Sajjan <vikas.sajjan@samsung.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
[kgene.kim@samsung.com: fixed coding style]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>