ARM: 9092/1: xen: Register with kernel restart handler

Register with kernel restart handler instead of setting arm_pm_restart
directly.

Select a high priority of 192 to ensure that default restart handlers
are replaced if Xen is running.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>