mm/Kconfig: make CONFIG_MEMORY_HOTPLUG depend on CONFIG_SPARSEMEM_VMEMMAP

Ever since commit f8f03eb5f0f9 ("mm: stop making SPARSEMEM_VMEMMAP
user-selectable"), an architecture that supports CONFIG_SPARSEMEM_VMEMMAP
(by selecting SPARSEMEM_VMEMMAP_ENABLE) can no longer enable
CONFIG_SPARSEMEM without CONFIG_SPARSEMEM_VMEMMAP.

Right now, CONFIG_MEMORY_HOTPLUG is guarded by CONFIG_SPARSEMEM.

However, CONFIG_ARCH_ENABLE_MEMORY_HOTPLUG is only enabled by
* arm64: which selects SPARSEMEM_VMEMMAP_ENABLE
* loongarch: which selects SPARSEMEM_VMEMMAP_ENABLE
* powerpc (64bit): which selects SPARSEMEM_VMEMMAP_ENABLE
* riscv (64bit): which selects SPARSEMEM_VMEMMAP_ENABLE
* s390 with SPARSEMEM: which selects SPARSEMEM_VMEMMAP_ENABLE
* x86 (64bit): which selects SPARSEMEM_VMEMMAP_ENABLE

So, we can make CONFIG_MEMORY_HOTPLUG depend on CONFIG_SPARSEMEM_VMEMMAP
without affecting any setups.

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-4-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>