arm/arm64: smp: rename secondary_halt to do_idle

Also prepare the newly renamed function to come out of idle
and use sev/wfe rather than a busy wait in smp_run().

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20170601135002.26704-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>