Allow Open Firmware syscons to attach to devices without an "address"
property such as those found on some real and emulated IBM systems. The
approach, which is taken from Linux, is to scan through the PCI bars
until we find one large enough to contain the linear framebuffer and
which is ideally prefetchable if no "address" property can be found.
This makes the graphical console work with the pSeries target in QEMU.

Approved by:	re (delphij)