cpu/stm32: move stm32l5 default PLL N to cpu
cpu/stm32: move stm32l5 default PLL N to cpu

authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT