drivers/clock_control: stm32u5: Factorize bus prescalers settings
drivers/clock_control: stm32u5: Factorize bus prescalers settings

Move prescaler settings to the clock_control_init function.
At this step they will be set up twice in PLL case, this will
be fixed in a next step.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,galak/zephyr