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 <[email protected]>
T:	git git://github.com/hnaz/linux-mm.git

[[email protected]: add ozlabs.org quilt trees]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Song Liu <[email protected]>
Acked-by: Andrew Morton <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: liu-song-6

MAINTAINERS

@@ -10519,8 +10519,12 @@ F:	mm/memblock.c
 F:	Documentation/core-api/boot-time-mm.rst
 
 MEMORY MANAGEMENT
+M:	Andrew Morton <[email protected]>
 L:	[email protected]
 W:	http://www.linux-mm.org
+T:	quilt https://ozlabs.org/~akpm/mmotm/
+T:	quilt https://ozlabs.org/~akpm/mmots/
+T:	git git://github.com/hnaz/linux-mm.git
 S:	Maintained
 F:	include/linux/mm.h
 F:	include/linux/gfp.h