PM / devfreq: exynos-ppmu: Staticize event list

The ppmu_events array is accessed only in this compilation unit so it
can be made static.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>