Register with kernel restart handler instead of setting arm_pm_restart directly.

This patch also addresses the following compile warning.

drivers/power/reset/xgene-reboot.c: In function 'xgene_reboot_probe':
drivers/power/reset/xgene-reboot.c:77:17: warning:
	assignment from incompatible pointer type [enabled by default]

The warning was due to a mismatch between the type of arm_pm_restart
and the restart function.

Cc: Loc Ho <lho@apm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>