MAINTAINERS: add tree entry to mm init block

Link: https://lkml.kernel.org/r/aFLubPfiO5hqfhCe@kernel.org


Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>