dma-mapping: don't store redundant offsets

A bus_dma_region necessarily stores both CPU and DMA base addresses for
a range, so there's no need to also store the difference between them.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>