power: Add APM X-Gene system reboot driver

Add APM X-Gene SoC system reboot driver. This driver handles only system
reboot. System shutdown is board specific and can be handled by board
driver or GPIO based shutdown driver.

Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Kumar Sankaran <ksankaran@apm.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>