cpu/sam0_common: uart: wait for synchronisation done after powerup
cpu/sam0_common: uart: wait for synchronisation done after powerup

Otherwise we lose settings if we write to it right after uart_poweron()

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