Implement usb_device for rp2040
Limitations
- RP2040-E5 (Device enumeration workaround) is not implemented
- Suspend/resume is not implemented
- VBus detection is not tested and may not be fully implemented
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.
Sorry this is a large commit :(
This adds support for input pins, including pulling them high or low.
It also adds two examples: the start of a classic blinky LED example, and an example for reading input.