Fix "undefined reference to `arch_add_memory'" on sparc64 allmodconfig.

sparc64 doesn't support memory hotplug.  But we want it to support
sparsemem.
Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>