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.
This newer peripheral supports DMA (through EasyDMA) and should
generally be faster. Importantly for some operations: interrupts (within
255 byte buffers) will not interfere with the SPI transfer.
The nrf52 series is all very similar and copying the code only makes it
harder to maintain the code or to add more chips in the nrf52 series
(for example, the nrf52833 as used in the micro:bit v2).
This commit also has a small improvement regarding pins: it now includes
chip-level pin names (P0.00, P0.01, etc) to the machine package.
