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/include/asm/acpi.h b/arch/x86/include/asm/acpi.h
index b31bf97..2dfac58 100644
--- a/arch/x86/include/asm/acpi.h
+++ b/arch/x86/include/asm/acpi.h
@@ -111,7 +111,7 @@
 }
 
 /* Low-level suspend routine. */
-extern int acpi_suspend_lowlevel(void);
+extern int (*acpi_suspend_lowlevel)(void);
 
 /* Physical address to resume after wakeup */
 #define acpi_wakeup_address ((unsigned long)(real_mode_header->wakeup_start))