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>
diff --git a/arch/x86/kernel/acpi/sleep.h b/arch/x86/kernel/acpi/sleep.h
index 67f59f8..c9c2c98 100644
--- a/arch/x86/kernel/acpi/sleep.h
+++ b/arch/x86/kernel/acpi/sleep.h
@@ -15,3 +15,5 @@
 extern void wakeup_long64(void);
 
 extern void do_suspend_lowlevel(void);
+
+extern int x86_acpi_suspend_lowlevel(void);