ci: Enable Dangerbot

Initial CI pipeline with Dangerbot enabled.

The danger-review job is the only job running in the .pre stage at the
moment.
Using a dummy job to run in the Setup stage to make the pipeline
visible.

Changelog: other
Signed-off-by: Jingzhao Ni <Jingzhao.Ni@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>