x86, acpi, tboot: Have a ACPI os prepare sleep instead of calling tboot_sleep.

The ACPI suspend path makes a call to tboot_sleep right before
it writes the PM1A, PM1B values. We replace the direct call to
tboot via an registration callback similar to __acpi_register_gsi.

CC: Len Brown <len.brown@intel.com>
Acked-by: Joseph Cihula <joseph.cihula@intel.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
[v1: Added __attribute__ ((unused))]
[v2: Introduced a wrapper instead of changing tboot_sleep return values]
[v3: Added return value AE_CTRL_SKIP for acpi_os_sleep_prepare]
Signed-off-by: Tang Liang <liang.tang@oracle.com>
[v1: Fix compile issues on IA64 and PPC64]
[v2: Fix where __acpi_os_prepare_sleep==NULL and did not go in sleep properly]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/kernel/tboot.c

@@ -297,6 +297,12 @@ void tboot_sleep(u8 sleep_state, u32 pm1a_control, u32 pm1b_control)
 
 	tboot_shutdown(acpi_shutdown_map[sleep_state]);
 }
+static int tboot_sleep_wrapper(u8 sleep_state, u32 pm1a_control,
+			       u32 pm1b_control)
+{
+	tboot_sleep(sleep_state, pm1a_control, pm1b_control);
+	return 0;
+}
 
 static atomic_t ap_wfs_count;
 
@@ -345,6 +351,8 @@ static __init int tboot_late_init(void)
 
 	atomic_set(&ap_wfs_count, 0);
 	register_hotcpu_notifier(&tboot_cpu_notifier);
+
+	acpi_os_set_prepare_sleep(&tboot_sleep_wrapper);
 	return 0;
 }