ci: Add missing rule for triggering pipeline for default branch

Add missing top-level rule to trigger a pipeline when the default branch
is pushed (i.e. a merge request is merged).

Note that the templates `.gitlint-check` and `.sync-public` already
contain their own rules, ensuring they are only run under the right
conditions such as gitlint on merge requests only and syncing on default
branch only.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>