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>

Author: apopple-nvidia

MAINTAINERS

@@ -15744,6 +15744,7 @@ R:	Rakie Kim <rakie.kim@sk.com>
 R:	Byungchul Park <byungchul@sk.com>
 R:	Gregory Price <gourry@gourry.net>
 R:	Ying Huang <ying.huang@linux.alibaba.com>
+R:	Alistair Popple <apopple@nvidia.com>
 L:	linux-mm@kvack.org
 S:	Maintained
 W:	http://www.linux-mm.org