[PATCH] solve config broken: undefined reference to `online_page'

Memory hotplug code of i386 adds memory to only highmem.  So, if
CONFIG_HIGHMEM is not set, CONFIG_MEMORY_HOTPLUG shouldn't be set.
Otherwise, it causes compile error.

In addition, many architecture can't use memory hotplug feature yet.  So, I
introduce CONFIG_ARCH_ENABLE_MEMORY_HOTPLUG.

Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>