PM / devfreq: event: Remove incorrect property in exynos-ppmu DT binding

The exynos-ppmu driver is only a clock consumer and not a clock provider
but its Device Tree binding listed #clock-cells as an optional property.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>