PM: Introduce new PMSG_POWEROFF event

PM: Introduce new PMSG_POWEROFF event

PMSG_POWEROFF will be used for the PM core to allow differentiating between
a hibernation or shutdown sequence when re-using callbacks for common code.

Hibernation is started by writing a hibernation method (such as 'platform'
'shutdown', or 'reboot') to use into /sys/power/disk and writing 'disk' to
/sys/power/state.

Shutdown is initiated with the reboot() syscall with arguments on whether
to halt the system or power it off.

Tested-by: Eric Naim <dnaim@cachyos.org>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/20251112224025.2051702-2-superm1@kernel.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>