dts/bindings: clock: cpu2-prescaler is optional on stm32wl

STM32WL could be either single core (stm32wlex) or
dual core (stm32wl5/4x).
Make CPU2 prescaler an optional property (while it will
be required if inherited from st,stm32wb-rcc.yaml.

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