ppc/pnv: add phb-id/chip-id PnvPHB4RootBus properties

ppc/pnv: add phb-id/chip-id PnvPHB4RootBus properties

The same rationale provided in the PHB3 bus case applies here.

Note: we could have merged both buses in a single object, like we did
with the root ports, and spare some boilerplate. The reason we opted to
preserve both buses objects is twofold:

- there's not user side advantage in doing so. Unifying the root ports
presents a clear user QOL change when we enable user created devices back.
The buses objects, aside from having a different QOM name, is transparent
to the user;

- we leave a door opened in case we want to increase the root port limit
for phb4/5 later on without having to deal with phb3 code.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220811163950.578927-3-danielhb413@gmail.com>