submodule: bump openthread from a470e8b to 555454c

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

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

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

Author: dependabot[bot]