ste_dma40: implement support for scatterlist to scatterlist copy

Now that the DMAEngine API has support for scatterlist to scatterlist
copy, implement support for the STE DMA40 DMA controller.

Cc: Linus Walleij <linus.ml.walleij@gmail.com>
Acked-by: Per Fridén <per.friden@stericsson.com>
Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>