ppc/8xxx: Refactor code to determine if PCI is enabled & agent/host

Refactor the code into a simple bitmask lookup table that determines if
a given PCI controller is enabled and if its in host/root-complex or
agent/end-point mode.

Each processor in the PQ3/MPC86xx family specified different encodings
for the cfg_host_agt[] and cfg_IO_ports[] boot strapping signals.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>