submodule: bump openthread from `5c8a9cb` to `515c3d3` (#307)
submodule: bump openthread from `5c8a9cb` to `515c3d3` (#307)

Bumps [openthread](https://github.com/openthread/openthread) from `5c8a9cb` to `515c3d3`.
- [Commits](https://github.com/openthread/openthread/compare/5c8a9cb5bc28ce6f794c0d8db7189b1f2dd586ba...515c3d3d80fe374011cb0577e4fbc5ad6393be45)

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

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-nrf528xx,openthread/ot-nrf528xx