Feature: FTDI structure clarity & documentation

[dragonmux]

Detailed description

This PR sets out to clean up the FTDI port state structure, document it, document where to find the MPSSE protocol and command documentation, and clean up the use of that structure.

This was spurred on when helping @lethalbit figure out how to describe the Kria SOM KV260's FTDI device pinout and getting caught up on what all the bits meant and did, resulting in us having to read the source and FTDI documentation to figure it out.

Tested against Tigard

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[amyspark]

😹 Hey! Just two nits I found while reading it?

[esden]

LGTM, thanks! :)