Using extmod/machine_pwm.c for the Python bindings and the existing
softpwm.c driver, by just adding the interface.
Properties:
- Frequency range 1-3906 Hz.
- All PWM outputs run at the same frequency but can have different duty
cycles.
- Limited to the P0.x pins.
Since it uses the existing softpwm.c mechanism, it will be affected by
playing music with the music class.
