* Implement GPIO override
Interrupt status, input, output enable and output can be overridden.
See documentation for GPIO0_CTRL.
* Fix override enum names
* Use override_value instead of raw identifier
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.