arm/arm64: psci: avoid calling halt directly

Eventually we'll allow calling functions on cpus more than once,
to allow multiple subtests to run in a single execution. But
that'll only work if the subtests don't halt the cpus...

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20170529135804.22891-4-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>