MAINTAINERS: add a section for policy documents

MAINTAINERS: add a section for policy documents

We don't update these often but now at least we have a few like minded
individuals keeping reviewers eye out for changes.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230403134920.2132362-5-alex.bennee@linaro.org>