MAINTAINERS: add memory mapping entry with reviewers

Recently there have been a number of patches which have affected various
aspects of the memory mapping logic as implemented in mm/mmap.c where it
would have been useful for regular contributors to have been notified.

Add an entry for this part of mm in particular with regular contributors
tagged as reviewers.

Link: https://lkml.kernel.org/r/20240220064410.4639-1-lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>