gitlab-ci.yml: Add jobs to build OpenSBI firmware binaries

gitlab-ci.yml: Add jobs to build OpenSBI firmware binaries

Add two GitLab jobs to build the OpenSBI firmware binaries.

The first job builds a Docker image with the packages requisite
to build OpenSBI, and stores this image in the GitLab registry.
The second job pulls the image from the registry and builds the
OpenSBI firmware binaries.

The docker image is only rebuilt if the GitLab YAML or the
Dockerfile is updated. The second job is only built when the
roms/opensbi/ submodule is updated, when a git-ref starts with
'opensbi' or when the last commit contains 'OpenSBI'. The files
generated are archived in the artifacts.zip file.

With OpenSBI v0.6, it took 2 minutes 56 seconds to build
the docker image, and 1 minute 24 seconds to generate the
artifacts.zip with the firmware binaries (filesize: 111KiB).

See: https://gitlab.com/lbmeng/qemu/pipelines/120520138

Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>