ppc/pnv: turn PnvPHB3 into a PnvPHB backend

ppc/pnv: turn PnvPHB3 into a PnvPHB backend

We need a handful of changes that needs to be done in a single swoop to
turn PnvPHB3 into a PnvPHB backend.

In the PnvPHB3, since the PnvPHB device implements PCIExpressHost and
will hold the PCI bus, change PnvPHB3 parent to TYPE_DEVICE. There are a
couple of instances in pnv_phb3.c that needs to access the PCI bus, so a
phb_base pointer is added to allow access to the parent PnvPHB. The
PnvPHB3 root port will now be connected to a PnvPHB object.

In pnv.c, the powernv8 machine chip8 will now hold an array of PnvPHB
objects.  pnv_get_phb3_child() needs to be adapted to return the PnvPHB3
backend from the PnvPHB child. A global property is added in
pnv_machine_power8_class_init() to ensure that all PnvPHBs are created
with phb->version = 3.

After all these changes we're still able to boot a powernv8 machine with
default settings. The real gain will come with user created PnvPHB
devices, coming up next.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220624084921.399219-4-danielhb413@gmail.com>