MIPS: Allow MAX_DMA32_PFN to be overridden.

DMA mapping may reduce the usable physical address range usable for
32-bit DMA.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1633/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>