MAINTAINERS: add linux-mm@ list to Kexec Handover

MAINTAINERS: add linux-mm@ list to Kexec Handover

Along with kexec, KHO also has parts dealing with memory management, like
page/folio initialization, memblock, and preserving/unpreserving memory
for next kernel.  Copy linux-mm@ to KHO patches so the right set of eyes
can look at changes to those parts.

Link: https://lkml.kernel.org/r/20250613131917.4488-1-pratyush@kernel.org
Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: SeongJae Park <sj@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>