dmaengine: dma40: Add support to split up large elements

The maximum transfer size of the stedma40 is (64k-1) x data-width.
If the transfer size of one element exceeds this limit
the job is split up and sent as linked transfer.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>