power/reset: arm-versatile: Register with kernel restart handler

Register with kernel restart handler instead of setting arm_pm_restart
directly. Select high priority since the restart handler is instantiated
through devicetree, indicating that it should be used if configured.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sebastian Reichel <sre@kernel.org>