[ACPI] ACPI poweroff fix

Register an "acpi" system device to be notified of shutdown preparation.
This depends on CONFIG_PM

http://bugzilla.kernel.org/show_bug.cgi?id=4041



Signed-off-by: Alexey Starikovskiy <alexey.y.starikovskiy@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>