metag: Add MAINTAINERS entry

Add MAINTAINERS entry for the metag architecture port.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Joe Perches <joe@perches.com>
Cc: "David S. Miller" <davem@davemloft.net>