ehea: Fix memory hotplug support

This patch implements the memory notifier to update the busmap
instantly instead of rebuilding the whole map. This is necessary
because walk_memory_resource provides different information than
required during memory hotplug.

Signed-off-by: Hannes Hering <hering2@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>