drivers/clock: stm32wl config restructure cpu2 prescaler assignment

This commmit restructures the clock_controller code such that the cpu2
prescler assignment later can be excluded for single core socs.

The stm32wl mcu line has variants with a single cortex-m4 core
(stm32wle5), therefore the prescaler for the second clock should
only be set for dual core socs.
This commit still checks for the complete series
(CONFIG_SOC_SERIES_STM32WLX) as the single core variants are not
yet introduced, later the condition should check for a flag like
CONFIG_SOC_STM32WL5X instead.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>