dt-bindings: stm32: syscon: Add clock support

STM32 system configuration controller registers needs to be clocked.
Document clock support on stm32-syscon.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>