Add GitHub Actions to mirror GitLab repo

The SCP-firmware GitHub repo is now a read-only mirror. A new GitHub
Actions is added to mirror the GitHub repo to SCP-firmware GitLab repo.
The action will fetch GitLab main branch and force push to GitHub
master branch. This will run every day at 8 AM.

Signed-off-by: Wen Ping Teh <wenping.teh@arm.com>
Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: Ie58b997ee0a5de3c7c20c6f73dd85d589e2d40ef