dts/bindings: Mark #clock-cells as optional in nxp,kinetis-sim.yaml

The nxp,kinetis-sim is shared by 2 SoC familes and on one of them
its not used for clocks.  As such that SoC will not have a #clock-cells
property so mark it optional.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>