hw/ppc/spapr: Adjust firmware name for PCI bridges

SLOF uses "pci" as name for PCI bridges nodes in the device tree instead
of "pci-bridges", so booting via bootindex from a device behind a PCI
bridge currently does not work since QEMU passes the wrong name in the
"qemu,boot-list" property. Fix it by changing the name of the PCI bridge
nodes to "pci" instead.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1459170
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
