hw/ppc: deprecate the machine type 'prep', replaced by '40p'

hw/ppc: deprecate the machine type 'prep', replaced by '40p'

- prep machine is a fictional machine, so has no specifications. Which
  devices can be changed/added/removed without impact? Are interrupts
  correctly mapped?
- prep firmware (OHW) has support only for IDE drives (no SCSI).
  Booting from IDE has been broken approximatively 3 years ago, and nobody complained.
- OHW is limited on IDE boot to a specific set of OS loaders.
  These operating systems are of the 2004 time frame.
- OHW can use -kernel. Linux kernel freezes a long time after PS/2 mouse
  detection, and then screen becomes garbage. This was already broken in
  QEMU v2.7, 2 years ago, and nobody complained.

On the other side:
- 40p is a real machine, so emulation can be checked against
  hardware specifications
- OpenBIOS has support for SCSI block devices, including 40p LSI adapter
- OpenBIOS can start mostly all Linux kernels (including recent ones)
  and recent operating system (like NetBSD 7.1.2)

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
[dwg: Drop prep from boot-serial test to avoid deprecation warnings]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>