DEC: Document the R4k MB ASIC mini interrupt controller

There's an alternative 5-line mini interrupt controller in the R4k MB ASIC
used on the KN04 and KN05 CPU daughtercards.  The controller is cascaded
from the CPU interrupt input that would be used for the Halt button on the
corresponding R3k systems.  This change documents the findings so far.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/6706/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>