MAINTAINERS: update information for "MEMORY MANAGEMENT"

I was trying to find the mm tree in MAINTAINERS by searching "Morton".
Unfortunately, I didn't find one.  And I didn't even locate the MEMORY
MANAGEMENT section quickly, because Andrew's name was not listed there.

Thanks to Johannes who helped me find the mm tree.

Let save other's time searching around by adding:

M:	Andrew Morton <akpm@linux-foundation.org>
T:	git git://github.com/hnaz/linux-mm.git

[akpm@linux-foundation.org: add ozlabs.org quilt trees]
Link: http://lkml.kernel.org/r/20191030202217.3498133-1-songliubraving@fb.com
Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>