drivers/clock_control: stm32 common: Factorize freq bus setting

Factorize setting of frequency for busses.
Additionally, factorize SysCoreClock update.

The operations are now done twice in case of PLL since they are part
of LL utils PLL configuration function, but they are removed in next
commits.

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