m68k: Add an entry for the NeXTcube machine to the MAINTAINERS file

I don't have much clue about the NeXT hardware, but at least I know now
the source files a little bit, so I volunteer to pick up patches and send
PULL requests for them until someone else with more knowledge steps up
to do this job instead.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190831074519.32613-7-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>