This commit refactors PWM support in the machine package to be more
flexible. The new API can be used to produce tones at a specific
frequency and control servos in a portable way, by abstracting over
counter widths and prescalers.
* machine/samd51: pin method cleanups.
- Use bit-math to select the group in Pin methods.
- Move Pin methods to atsamd51. They are not chip-specific, they apply to the whole atsamd51 family.
- Move the group/pin-id calculation into a helper method.
- Add a Pin.Toggle() method.
