MAINTAINERS: add MEMORY HOT(UN)PLUG section and add David as reviewer

There are certainly a lot more files that partially fall into the memory
hot(un)plug category, including parts of mm/sparse.c, mm/page_isolation.c
and mm/page_alloc.c.  Let's only add what's almost completely memory
hot(un)plug related.

Add myself as reviewer so it's easier for contributors to figure out
whom to CC.

Link: https://lkml.kernel.org/r/20220610101258.75738-1-david@redhat.com
Link: https://lkml.kernel.org/r/YqlaE/LYHwB0gpaW@localhost.localdomain


Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>