scripts/checkpatch: use new hook for MAINTAINERS update check

scripts/checkpatch: use new hook for MAINTAINERS update check

When seeing a new/deleted/renamed file we check to see if MAINTAINERS
is updated, but we don't give the user a list of files affected, as
we don't want to repeat the same warning many times over.

Using the new file list hook, we can give a single warning at the
end with a list of filenames included.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>