get_maintainer.pl: add .get_maintainer.conf

Since commit e57c7c5c ("get_maintainer.pl: update from Linux kernel
v5.13-rc6") only the top level MAINTAINERS file is used. This is because
this commit (accidentally) disabled the search of MAINTAINERS files
(find_maintainer_files is set to 0 again). Before that, commit
b79372ae ("scripts/get_maintainer.pl: enable find_maintainer_files")
explicitly enabled that feature. Nowadays, we also have to set
maintainer_path to a directory.

To fix it and enable recursive search of MAINTAINERS, create a
configuration file to set these two variables.

Signed-off-by: Michael Walle <michael@walle.cc>