boards/nucleo-f3xx: adapt clock config

boards/nucleo-f030/include/periph_conf.h

boards/nucleo-f070/include/periph_conf.h

boards/nucleo-f072/include/periph_conf.h

boards/nucleo-f091/include/periph_conf.h

boards/nucleo-f103/include/periph_conf.h

boards/nucleo-f302/include/periph_conf.h

boards/nucleo-f303/include/periph_conf.h

boards/nucleo-f334/include/periph_conf.h

boards/nucleo144-f303/include/periph_conf.h

boards/nucleo32-f031/include/periph_conf.h

boards/nucleo32-f042/include/periph_conf.h

boards/nucleo32-f303/include/periph_conf.h

boards/stm32f0discovery/include/periph_conf.h

boards/stm32f3discovery/include/periph_conf.h