[coreboot] Max pci bus number?

[Kevin O'Connor]

How can I find the maximum PCI bus number of the system?

The PCI bios calls need to be able to return this information.  It
also helps when scanning for pci devices, because then we don't have
to check for all 256 possible buses.

Right now, SeaBIOS has a compile time definition for the max, but that
is awkward and it can lead to hard to debug failures.

Ideas?
-Kevin