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>