Add POWER9 cpu to list of spapr core models which allows it to be specified
as the cpu model for a pseries guest (e.g. -machine pseries -cpu POWER9).

This now allows a POWER9 cpu to boot to userspace in tcg emulation for a
pseries machine with a legacy kernel.
Signed-off-by: NSuraj Jitindar Singh <sjitindarsingh@gmail.com>
Reviewed-by: NDavid Gibson <david@gibson.dropbear.id.au>
Acked-by: NBalbir Singh <bsingharora@gmail.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>