MIPS: Add a platform hook for swiotlb setup.

This allows platforms that are using the swiotlb to initialize it.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1638/


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