[MIPS] No I/O ports on the DECstation

 There are no I/O ports on the DECstation whatsoever in any configuration 
as neither the CPU nor the peripheral buses used have a concept of such.

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