arm64: dts: Add poweroff device nodes for APM X-Gene 2 platform

This patch adds:
 + poweroff button device node to support poweroff by
   power button on APM X-Gene 2 Merlin platform.
 + syscon poweroff device node to support poweroff feature
   on APM X-Gene 2 Merlin platform

Signed-off-by: Duc Dang <dhdang@apm.com>