submodule: bump openthread from a57d927 to e7535f7 (#868)

Bumps [openthread](https://github.com/openthread/openthread) from `a57d927` to `e7535f7`.
- [Commits](openthread/openthread@a57d927...e7535f7)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>