prep: add IBM RS/6000 7020 (40p) machine emulation

Machine supports both Open Hack'Ware and OpenBIOS.
Open Hack'Ware is the default because OpenBIOS is currently unable to boot
PReP boot partitions or PReP kernels.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
[dwg: Correct compile failure with KVM located by Thomas Huth]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
