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 <[email protected]>
Reviewed-by: Daniel Kiper <[email protected]>
Reviewed-by: Tang Chen <[email protected]>