PM: ACPI: reboot: Reinstate S5 for reboot

Commit d60cd063 ("PM: ACPI: reboot: Use S5 for reboot") caused Dell
PowerEdge r440 hangs at reboot.

The issue is fixed by commit 2ca1c94c ("tg3: Disable tg3 device on
system reboot to avoid triggering AER"), so use the new sysoff API to
reinstate S5 for reboot on ACPI-based systems.

Using S5 for reboot is default behavior under Windows: "A full shutdown
(S5) occurs when a system restart is requested" [1].

Link: https://docs.microsoft.com/en-us/windows/win32/power/system-power-state

 # [1]
Suggested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>