change trim units from absolute usec to normalized values

Tools/px4params/xmlout.py

src/drivers/drv_pwm_output.h

src/drivers/px4fmu/fmu.cpp

src/drivers/px4fmu/px4fmu_params.c

src/modules/px4iofirmware/mixer.cpp

src/modules/px4iofirmware/px4io.h

src/modules/px4iofirmware/registers.c

src/modules/systemlib/mixer/mixer.h

src/modules/systemlib/mixer/mixer_group.cpp

src/modules/systemlib/pwm_limit/pwm_limit.c

src/modules/systemlib/pwm_limit/pwm_limit.h