MAINTAINERS: List file memory-barriers.txt within the LKMM entry

We now have a shiny new Linux-kernel memory model (LKMM) and the old
tried-and-true Documentation/memory-barrier.txt.  It would be good to
keep these automatically synchronized, but in the meantime we need at
least let people know that they are related.  Will suggested adding the
Documentation/memory-barrier.txt file to the LKMM maintainership list,
thus making the LKMM maintainers responsible for both the old and the new.
This commit follows Will's excellent suggestion.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akiyks@gmail.com
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: nborisov@suse.com
Cc: npiggin@gmail.com
Cc: stern@rowland.harvard.edu
Link: http://lkml.kernel.org/r/1519169112-20593-3-git-send-email-paulmck@linux.vnet.ibm.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>