WIP: ws2812: add support for 125MHz for RP2040

[bgould]

This adds support for 125Mhz CPU frequency to WS2812

FYI I accidentally pushed and then reverted this change to dev; was sure of the best way to clean that up, hope I did it correctly.

accidental push: b4eb406
revert: 0ced126

[aykevl]

This looks good to me. Is this still a WIP pull request?

[bgould]

@aykevl I'm getting this asm error when trying to compile using RP2040 (using this branch in TinyGo: https://github.com/tinygo-org/tinygo/tree/macropad-rp2040)

bcg@bcg-x12:~/src/drivers$ tinygo version
tinygo version 0.23.0-dev linux/amd64 (using go version go1.17.3 and LLVM version 13.0.1)
bcg@bcg-x12:~/src/drivers$ tinygo build -o test.hex -target=macropad-rp2040 ./examples/ws2812
error: <inline asm>:278:4: out of range pc-relative fixup value
          bne.n 1b                       @ [1/3] send_bit
          ^

(haven't added a smoke test for this to drivers yet, was going to do that after macropad-rp2040 is merged in TinyGo)

[aykevl]

Here is a fix: #391