feat(boards): Add ZMK specific tweaks for upstream rp2040-zero board

[nmunnich]

RP2040-zero is a popular enough board that we should have these tweaks in-tree.

[petejohanson]

I think we should hold off on this pending the discussions in #3145

[peterjc]

I'm working on a tester_rpi_zero for the Waveshare PR2040-Zero or same pin-out Waveshare RP2350-Zero, abusing the rpi_pico board for now with &gpio0 29 etc since this exposes a different set of GPIOs (all of GP0 to GP29 inclusive except GP16 used for the RGB LED).

Should I make a PR with it now, or wait on the rp2040_zero board in ZMK?

[peterjc]

As per @nmunnich via Discord, I'll wait on this PR, and then submit the tester using rp2040-zero as the board.

Note to self: Move the back pads to the left to match front view, or off to one side perhaps?

[peterjc]

My Raspberry Pi Zero tester is currently on the branch for peterjc/zmk-keyboard-graph-theory#12 - originally that was for testing my Raspberry Pi PR2040 designs with the RP2350, but is now cleaned up to just focus on the new RP2040-Zero tester.

I'll turn that into a ZMK PR once this issue is done (and it should then use the new rp2040-zero board rather than abusing the rpi_pico board).

[petejohanson]

Should be rebased now that #3145 is in.