PM / devfreq: exynos-ppmu: ppmu_events array should not be NULL terminated

The rest of the code uses ARRAY_SIZE to count the number of entries in
ppmu_events array. The NULL terminated entry makes ARRAY_SIZE return
off-by-one value.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>