x86 / ACPI / sleep: Provide registration for acpi_suspend_lowlevel.

Which by default will be x86_acpi_suspend_lowlevel.
This registration allows us to register another callback
if there is a need to use another platform specific callback.

Signed-off-by: Liang Tang <liang.tang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Ben Guthro <benjamin.guthro@citrix.com>
Acked-by: "H. Peter Anvin" <hpa@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>