mm: memory hotplug with an existing resource

mm: memory hotplug with an existing resource

Add add_memory_resource() to add memory using an existing "System RAM"
resource.  This is useful if the memory region is being located by
finding a free resource slot with allocate_resource().

Xen guests will make use of this in their balloon driver to hotplug
arbitrary amounts of memory in response to toolstack requests.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Tang Chen <tangchen@cn.fujitsu.com>
Conflicts:
	include/linux/memory_hotplug.h
	mm/memory_hotplug.c

(cherry picked from commit 62cedb9f135794ec26a93ae29e5f0231ab263c84)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>