dts/st: dtc v1.4.6 warnings: Missing property '#clock-cells' in node

To prepare to upcoming dtc v1.4.6, fix warnings in dts files.

This commit addresses the following warning:
Warning (clocks_property): Missing property '#clock-cells' in node
/soc/rcc@40021000 or bad phandle (referred from
/soc/pin-controller/gpio@40011800:clocks[0])

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>