mips/netlogic: remove swiotlb support

nlm_swiotlb_dma_ops is unused code, so the whole swiotlb support is dead.
If it gets resurrected at some point it should use the generic
swiotlb_dma_ops instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian König <christian.koenig@amd.com>