power/reset: keystone: Register with kernel restart handler

Register with kernel restart handler instead of setting arm_pm_restart directly.

Move notifier registration to the end of the probe function to avoid having to
implement error handling.

Cc: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>