Initial implementation of PIO
Add all of the infrastructure required to include PIO programs and interact with PIO peripheral registers.
This was a massive effort between @devsnek and @henkkuli. It's great that it's now ready to go.
Co-authored-by: Gus Caplan <me@gus.host>
Bring in line with atsamd-hal GPIO v2
Copied as much as possible. Docs lifted mostly as-is.
Also add sample BSP for the Feather RP2040 in boards/feather_rp2040
May include a few random fixes from currently futile attempt to get doctests working.
dd a `SubsystemReset` trait which adds a `reset_bring_up` function to the
relevant PAC types to handle bringing subsystems out of reset.
Also, correct that the PLL and UART modules did not bring the relevant
subsystems out of reset and refactor the GPIO module to use the
SubsystemReset trait.
