x86: add memory hotremove config option

Impact: enable CONFIG_MEMORY_HOTREMOVE feature on x86. (default-off)

Memory hotremove functionality can currently be configured into
the ia64, powerpc, and s390 kernels.

This patch makes it possible to configure the memory hotremove
functionality into the x86 kernel as well.

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Gary Hade <garyhade@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>