ppc/pnv: introduce new skiboot platform properties

Newer skiboots (after 6.3) support QEMU platforms that have
characteristics closer to real OpenPOWER systems. The CPU type is used
to define the BMC drivers: Aspeed AST2400 for POWER8 processors and
AST2500 for POWER9s.

Advertise the new platform property names, "qemu,powernv8" and
"qemu,powernv9", using the CPU type chosen for the QEMU PowerNV
machine. Also, advertise the original platform name "qemu,powernv" in
case of POWER8 processors for compatibility with older skiboots.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190527071749.31499-1-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>