build(deps): bump third_party/cores/opentitan

Bumps [third_party/cores/opentitan](https://github.com/lowRISC/opentitan) from `eec48c4` to `f5e6ad8`.
- [Release notes](https://github.com/lowRISC/opentitan/releases)
- [Commits](lowRISC/opentitan@eec48c4...f5e6ad8)

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

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

Author: dependabot[bot]