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: NJavier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: NChanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: NMyungJoo Ham <myungjoo.ham@samsung.com>

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

@@ -17,7 +17,6 @@ Required properties:
 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.