MAINTAINERS: add entry for memblock

Add entry for memblock in MAINTAINERS file

Link: http://lkml.kernel.org/r/20190214093630.GC9063@rapoport-lnx


Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>