Allwinner software/hardware PWM

[biotinker]

What kind of new feature are you looking for?

• Software: PWM support on Allwinner CPUs. Currently gpio.PWM on allwinner simply returns: errors.New("not available on this CPU architecture")

Do you plan to:

• Contribute an initial driver: Yes
• Update https://github.com/periph/cmd to use the new functionality: Yes

I'm happy to go take a stab at implementing this. However, any background on why this isn't already implemented would be very helpful as a starting point.