PM: ACPI: reboot: Use S5 for reboot

After reboot, it's not possible to use hotkeys to enter BIOS setup
and boot menu on some HP laptops.

BIOS folks identified the root cause is the missing _PTS call, and
BIOS is expecting _PTS to do proper reset.

Using S5 for reboot is default behavior under Windows, "A full
shutdown (S5) occurs when a system restart is requested" [1], so
let's do the same here.

[1] https://docs.microsoft.com/en-us/windows/win32/power/system-power-states



Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
[ rjw: Subject edit ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>