MAINTAINERS: add Alistair as reviewer of mm memory policy

I'm particularly familiar with mm/migrate.c and especially
mm/migrate_device.c so add myself to MAINTAINERS.

Link: https://lkml.kernel.org/r/20250530014917.2946940-1-apopple@nvidia.com


Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>