This is a breaking change, making the hardware PWM on the nrf port
compatible with the other ports providing machine.PWM.
Frequency range 4Hz - ~5.4 MHz. The base clock range is 125kHz to 16 MHz,
and the divider range is 3 - 32767.
The hardware supports up to four outputs per PWM device with different duty
cycles, but only one output is (and was) supported.
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