[MIPS] DECstation: Document more MB ASIC register bits

 Document a few more register bits provided by the MB ASIC used on R4000SC
(KN04) and R4400SC (KN05) CPU daughtercards with the DECstation.  

 Reverse-engineered and not documented anywhere else to the best of my
knowledge.  Bit names appended to the last underscore the same as reported
by the firmware in register dumps.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>