MAINTAINERS: add maple tree mailing list

There is a mailing list for the maple tree development.  Add the list to
the maple tree entry of the MAINTAINERS file so patches will be sent to
interested parties.

Link: https://lkml.kernel.org/r/20230731175542.1653200-1-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

MAINTAINERS

@@ -12481,6 +12481,7 @@ F:	net/mctp/
 
 MAPLE TREE
 M:	Liam R. Howlett <Liam.Howlett@oracle.com>
+L:	maple-tree@lists.infradead.org
 L:	linux-mm@kvack.org
 S:	Supported
 F:	Documentation/core-api/maple_tree.rst