docs/system/target-mips.rst and configs/targets/mips* are not covered
in our MAINTAINERS file yet, so let's add them now.

Message-Id: <20221212171252.194864-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>