ACPI: power: Switch to sys-off handler API

Switch to sys-off API that replaces legacy pm_power_off callbacks,
allowing us to remove global pm_* variables and support chaining of
all restart and power-off modes consistently.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>