boards: fix PWM doxygen doc in periph_conf

boards/arduino-due/include/periph_conf.h

boards/frdm-k64f/include/periph_conf.h

boards/msba2/include/periph_conf.h

boards/msbiot/include/periph_conf.h

boards/mulle/include/periph_conf.h

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-f303/include/periph_conf.h

boards/nucleo-f334/include/periph_conf.h

boards/nucleo-f401/include/periph_conf.h

boards/nucleo-f446/include/periph_conf.h

boards/nucleo144-f207/include/periph_conf.h

boards/nucleo32-f042/include/periph_conf.h

boards/nucleo32-f303/include/periph_conf.h

boards/pba-d-01-kw2x/include/periph_conf.h

boards/stm32f3discovery/include/periph_conf.h

boards/stm32f4discovery/include/periph_conf.h