MIPS: Octeon: Adjust top of DMA32 zone.

On OCTEON, we reserve the last 256MB of 32-bit PCI address space, mapping
the RAM in this region at a high DMA address.  This makes memory in this
region unavailable for 32-bit DMA.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1634/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>