Merge pull request #4638 from gebart/pr/periph-pwm-api-change

boards/msba2/include/periph_conf.h

boards/samr21-xpro/include/periph_conf.h

boards/stm32f3discovery/include/periph_conf.h

cpu/kinetis_common/periph/pwm.c

cpu/lpc11u34/periph/pwm.c

cpu/lpc2387/periph/pwm.c

cpu/sam3/periph/pwm.c

cpu/samd21/periph/pwm.c

cpu/stm32f3/periph/pwm.c

cpu/stm32f4/periph/pwm.c

drivers/include/periph/dev_enums.h

drivers/include/periph/pwm.h

drivers/include/servo.h

drivers/servo/servo.c

tests/periph_pwm/main.c