ppc/pnv: turn PnvPHB4 into a PnvPHB backend

ppc/pnv: turn PnvPHB4 into a PnvPHB backend

Change the parent type of the PnvPHB4 device to TYPE_PARENT since the
PCI bus is going to be initialized by the PnvPHB parent. Functions that
needs to access the bus via a PnvPHB4 object can do so via the
phb4->phb_base pointer.

pnv_phb4_pec now creates a PnvPHB object.

The powernv9 machine class will create PnvPHB devices with version '4'.
powernv10 will create using version '5'. Both are using global machine
properties in their class_init() to do that.

These changes will benefit us when adding PnvPHB user creatable devices
for powernv9 and powernv10.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220624084921.399219-6-danielhb413@gmail.com>