ppc/pnv: add PnvPHB base/proxy device

ppc/pnv: add PnvPHB base/proxy device

The PnvPHB device is going to be the base device for all other powernv
PHBs. It consists of a device that has the same user API as the other
PHB, namely being a PCIHostBridge and having chip-id and index
properties. It also has a 'backend' pointer that will be initialized
with the PHB implementation that the device is going to use.

The initialization of the PHB backend is done by checking the PHB
version via a 'version' attribute that can be set via a global machine
property.  The 'version' field will be used to make adjustments based on
the running version, e.g. PHB3 uses a 'chip' reference while PHB4 uses
'pec'. To init the PnvPHB bus we'll rely on helpers for each version.
The version 3 helper is already added (pnv_phb3_bus_init), the PHB4
helper will be added later on.

For now let's add the basic logic of the PnvPHB object, which consists
mostly of pnv_phb_realize() doing all the work of checking the
phb->version set, initializing the proper backend, passing through its
attributes to the chosen backend, finalizing the backend realize and
adding a root port in the end.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220624084921.399219-3-danielhb413@gmail.com>