build(deps): bump third_party/litex from 95b310e to 644ef7e

Bumps [third_party/litex](https://github.com/enjoy-digital/litex) from `95b310e` to `644ef7e`.
- [Commits](enjoy-digital/litex@95b310e...644ef7e)

---
updated-dependencies:
- dependency-name: third_party/litex
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>