apb: add busA qdev property to PBM PCI bridge

As future sun4u PCI topologies place the ebus containing the in-built devices
behind a PCI bridge, add a busA property to the PBM PCI bridge that is then
used to allow IO accesses by default.

This allows early fw_cfg/NVRAM/serial access to occur even before OpenBIOS
has had a chance to configure the PCI bridges.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>