MIPS: Octeon: refactor swiotlb code

MIPS: Octeon: refactor swiotlb code

Share a common set of swiotlb operations, and to instead branch out in
__phys_to_dma/__dma_to_phys for the PCI vs non-PCI case.  Also use const
structures for the PCI methods so that attackers can't use them as
exploit vectors.

Signed-off-by: Christoph Hellwig <[email protected]>
Patchwork: https://patchwork.linux-mips.org/patch/19532/
Signed-off-by: Paul Burton <[email protected]>
Cc: Florian Fainelli <[email protected]>
Cc: David Daney <[email protected]>
Cc: Kevin Cernekee <[email protected]>
Cc: Jiaxun Yang <[email protected]>
Cc: Tom Bogendoerfer <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: [email protected]
Cc: [email protected]