[MIPS] Remove obsolete isa_slot_offset

 The isa_slot_offset variable and its __ISA_IO_base macro is not used
anywhere anymore.  It does not look like a decent interface per today's
standards either.  Remove both including all places of initialization.

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