ppc/pnv: Introduce a pnv-phb5 device to match root port

We already have the pnv-phb3 and pnv-phb4 devices for POWER8 and
POWER9 respectively. POWER10 uses version 5 of the PHB. It is very
close to the PHB4 from POWER9, at least in our model and we could
almost keep using the PHB4 model. However the matching root port
pnv-phb5-root-port is specific to POWER10 so to avoid confusion as
well as making it easy to introduce differences later, we create a
pnv-phb5 class, which is mostly an alias for pnv-phb4 for now.

With this patch, the command line for a user-created PHB on powernv10
becomes:
  -machine powernv10 -nodefaults -device pnv-phb5 -device pnv-phb5-root-port

Fixes: 623575e1 ("ppc/pnv: Add model for POWER10 PHB5 PCIe Host bridge")
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220310155101.294568-2-fbarrat@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>