Document the process by which a MAC addiress is selected.
Add accessor functions for core and thread information registers.
Reimplement platform_processor_id() in terms of the core and thread
information registers.
Add some sanity checks and warnings for validity of core and thread
information.
Retrieve the number of threads during attach and store it for later use
when routing interrupts.

The code to round-robin between target threads will be enabled at a later
date.
Allow multiple PIC devices.

For now, reach across to the right PIC directly.
Document the EBase and CDMM registers in MIPS32/64.
Document the BERI core and thread info registers at their new location.
Document atsectl.