ACPI: Add CPU hotplug support for processor device objects

acpi_processor_install_hotplug_notify() registers processor objects to
receive ACPI CPU hotplug event notifications. This patch additionally
registers processor device objects (ACPI0007) to receive the notifications
as well.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Len Brown <len.brown@intel.com>