ppc/pnv: Attach PHB3 root port device when defaults are enabled

This cleanups the PHB3 model a bit more since the root port is an
independent device and it will ease our task when adding user created
PHB3s.

pnv_phb_attach_root_port() is made public in pnv.c so it can be reused
with the pnv_phb4 root port later.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220105212338.49899-4-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>