Use empty_slot to reserve addresses for several unimplemented devices so they won't fault.
 - BPP (parallel port), DBRI (audio), SX (pixel processor), and vsimms (framebuffer)
OBP for SS-20 either assumes these devices exist or probes without expecting faults.
Signed-off-by: NBob Breuer <breuerr@mc.net>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>