power: reset: Add syscon poweroff device node for APM X-Gene platform

This patch adds syscon poweroff device node to support poweroff feature
on APM X-Gene Mustang platform

Signed-off-by: Tai Nguyen <ttnguyen@apm.com>
Tested-by: dann frazier <dann.frazier@canoncial.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>