mips,unicore32: swiotlb doesn't need sg->dma_length

Only mips and unicore32 select CONFIG_NEED_SG_DMA_LENGTH when building
swiotlb.  swiotlb itself never merges segements and doesn't accesses the
dma_length field directly, so drop the dependency.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: James Hogan <jhogan@kernel.org>