prep: add PReP System I/O

prep: add PReP System I/O

This device is a partial duplicate of System I/O device available in hw/ppc/prep.c
This new one doesn't have all the Motorola-specific registers.
The old one should be deprecated and removed with the 'prep' machine.

Partial documentation available at
ftp://ftp.software.ibm.com/rs6000/technology/spec/srp1_1.exe
section 6.1.5 (I/O Device Mapping)

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>