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 <[e-mail]>
Reviewed-by: Chanwoo Choi <[e-mail]>
Signed-off-by: MyungJoo Ham <[e-mail]>

Documentation/devicetree/bindings/devfreq/event/exynos-ppmu.txt

···
 Optional properties:
 - clock-names : the name of clock used by the PPMU, "ppmu"
 - clocks : phandles for clock specified in "clock-names" property
-- #clock-cells: should be 1.
 
 Example1 : PPMU nodes in exynos3250.dtsi are listed below.