Like acpi_os_prepare_sleep(), register a callback for use in systems
like tboot, and xen, which have system specific requirements outside
of ACPICA.  This mirrors the functionality in acpi_os_prepare_sleep(),
called from acpi_hw_sleep()
Signed-off-by: Ben Guthro <benjamin.guthro@citrix.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>