PSCI demands that SYSTEM_OFF must not return. While it seems like a
generic ATF bug that this is possible when a platform does not Implement
a corresponding handler, let's do that here until it's addressed
differently.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Change-Id: I4c08948b18bbfdc3a24214f2ae0fbad9e017ada1