USB device support

[r4gus]

I've ported the usb bulk example (found in the rp2040 datasheet) to Zig. The Rust implementation of Cliff Biffle with all the comments helped a lot.

I want to add support for usb interrupt endpoints before I open an pull request but let me know what you think.

Link to the usb device impl: https://github.com/r4gus/raspberrypi-rp2040/blob/2nd-usb-attempt/src/hal/usb.zig
Link to the example: https://github.com/r4gus/raspberrypi-rp2040/blob/2nd-usb-attempt/examples/usb_device.zig
Link to a python test script: https://github.com/raspberrypi/pico-examples/blob/master/usb/device/dev_lowlevel/dev_lowlevel_loopback.py

You can also get some debug output over uart

[ikskuh]

That sounds pretty damn cool! Feel free to open a draft PR so we can see updates :)