MAINTAINERS: notify the "Device Memory" community of memory hotplug changes

The recent episode of a warning regression in memremap_pages() [1]
highlights that relevant updates are being missed by folks that care about
core ZONE_DEVICE changes.  Yes, CXL folks should pay more attention to
linux-mm@, but it also would not hurt to copy linux-cxl@, where most
Device Memory folks hang out, on memory hotplug changes by default.

Link: http://lore.kernel.org/20251219123717.39330-1-john@groves.net [1]
Link: https://lkml.kernel.org/r/20251220000327.3502994-1-dan.j.williams@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Acked-by: John Groves <John@Groves.net>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>