We want to create only the absolutely minimal amount of devices when
running with -nodefaults. The root port is something that the machine
can boot up without. But, to do that, we need to provide a way for the
user to add them by hand.

This patch makes pnv-phb4-root-port user creatable and then uses the
pnv_phb_attach_root_port() helper to add a pnv_phb4_root_port only when
running with default settings.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220105212338.49899-5-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>