ppc/pnv/phb4: Add pervasive chiplet support to PHB4/5

ppc/pnv/phb4: Add pervasive chiplet support to PHB4/5

Each non-core chiplet on a chip has a "pervasive chiplet" unit and its
xscom register set. This adds support for PHB4/5.

skiboot reads the CPLT_CONF1 register in __phb4/5_get_max_link_width(),
which shows up as unimplemented xscom reads. Set a value in PCI CONF1
register's link-width field to demonstrate skiboot doing something
interesting with it.

In the bigger picture, it might be better to model the pervasive
chiplet type as parent that each non-core chiplet model derives from.
For now this is enough to get the PHB registers implemented and working
for skiboot, and provides a second example (after the N1 chiplet) that
will help if the design is reworked as such.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>