target/ppc: Add POWER9 DD2.0 model information

target/ppc: Add POWER9 DD2.0 model information

At the moment the only POWER9 model which is listed in qemu is v1.0 (aka
"DD1").  This is a very early (read, buggy) version which will never be
released to the public - it was included in qemu only for the convenience
of those doing bringup on the early silicon.  For bonus points, we actually
had its PVR incorrect in the table (0x004e0000 instead of 0x004e0100).  We
also never actually implemented the differences in behaviour (read, bugs)
that marked DD1 in qemu.

Now that we know the PVR for the substantially better v2.0 (DD2) chip,
include it and make it the default POWER9 in qemu.  For the time being we
leave the DD1 definition in place for the poor souls (read, me) who still
need to work with DD1 hardware.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>