documentation:
- Remove "mainbus" altogether.
- The new root is "plb" - the Processor Local Bus.
- Attached to this is the "opb" - the On-chip Peripheral Bus, to which
all the on-chip devices are attached (except the cpu and pci host
bridge).
- Port-specific code can pass an array of 'struct plb_dev' to
config_rootfound() to attach extra devices to the plb. The walnut
port attaches a "pbus" (Peripheral Bus) in here for the RTC and
pc keyboard controller to attach to.
There is still much 405GP specific code; the next round of changes will
generalise this to enable easier support for other 4xx CPUs.
simonb