submodule: bump openthread from a470e8b to 555454c (#926)

Bumps [openthread](https://github.com/openthread/openthread) from `a470e8b` to `555454c`.
- [Commits](openthread/openthread@a470e8b...555454c)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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