MIPS: consolidate the swiotlb implementations

MIPS: consolidate the swiotlb implementations

Octeon and Loongson share exactly the same code, move it into a common
implementation, and use that implementation directly from get_arch_dma_ops.

Also provide the expected dma-direct.h helpers directly instead of
delegating to platform dma-coherence.h headers.

Signed-off-by: Christoph Hellwig <[email protected]>
Patchwork: https://patchwork.linux-mips.org/patch/19534/
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]