MAINTAINERS: add Mikulas Patocka as a DM maintainer

Mikulas is a long-time contributor to the DM subsystem and has
effectively been a shadow DM maintainer.  It is time to formally
name Mikulas as a DM maintainer.

In practice this doesn't imply any process changes for DM maintenance
(Mikulas can still just feed changes and contribute review like
normal).

This change does allow the possibility for Mikulas to take on more
responsibility in actually sending DM changes upstream.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>