MAINTAINERS: Add section for Aarch64 GitLab custom runner

MAINTAINERS: Add section for Aarch64 GitLab custom runner

Add a MAINTAINERS section to cover the GitLab YAML config file
containing the jobs run on the custom runner sponsored by the
Works On Arm project [*].

[*] https://developer.arm.com/solutions/infrastructure/works-on-arm

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211116163226.2719320-1-f4bug@amsat.org>
Message-Id: <20211129140932.4115115-8-alex.bennee@linaro.org>