PM / devfreq: exynos-ppmu: fix error path in exynos_ppmu_probe()

iounmap() needs to be called in case of memory allocation
(for devfreq-event devices) failure.  Fix it.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>