kernel/reboot: Add SYS_OFF_MODE_RESTART_PREPARE mode

Add SYS_OFF_MODE_RESTART_PREPARE callbacks to be invoked before
a system restart.

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