thunderbolt: Import USB4 code

Add initial USB4 code written by Scott Long and originally passed on to
HPS (source: https://github.com/hselasky/usb4), minus the ICM code and
with some small fixes.

For context, older TB chips implemented the connection manager in
firmware (ICM) instead of in the OS (HCM), but maintaining the ICM code
would be a huge burden for not many chips.

Mostly completed work:

• Debug/trace framework.
• NHI controller driver.
• PCIe bridge driver.
• Router and config space layer handling (just reading in this commit).

Link to the email where Scott shared details about the initial USB4
work:

https://lists.freebsd.org/archives/freebsd-hackers/2024-July/003411.html

Glanced at by: emaste, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49450
Event: EuroBSDcon 2025