ppc: restrict the use of the rfi instruction

Power ISA 2.x has deleted the rfi instruction and rfid shoud be used
instead on cpus following this instruction set or later.

This will raise an invalid exception when rfi is used on such
processors: Book3S 64-bit processors.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
[clg: the required fix in openbios, commit b747b6acc272 ('ppc: use
      rfid when running under a CPU from the 970 family.'), is now
      merged in qemu under commit 5cebd885d0d2 ('Update OpenBIOS
      images to b747b6a built from submodule.') ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
