memhp: merge build_memory_devices() into build_memory_hotplug_aml()

It consolidates memory hotplug AML in one place within DSDT

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>