[MIPS] SB1250: Initialize io_map_base

Correctly initialize io_map_base for the SB1250 PCI controller as required
for proper iomap support.  Based on a proposal from Daniel Jacobowitz.

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