ppc/pnv: add pnv-phb-root-port device

ppc/pnv: add pnv-phb-root-port device

We have two very similar root-port devices, pnv-phb3-root-port and
pnv-phb4-root-port. Both consist of a wrapper around the PCIESlot device
that, until now, has no additional attributes.

The main difference between the PHB3 and PHB4 root ports is that
pnv-phb4-root-port has the pnv_phb4_root_port_reset() callback. All
other differences can be merged in a single device without too much
trouble.

This patch introduces the unified pnv-phb-root-port that, in time, will
be used as the default root port for the pnv-phb device.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220624084921.399219-7-danielhb413@gmail.com>