Improve slave/cyclic DMA engine documentation

Improve the documentation for the slave and cyclic DMA engine support
reformatting it for easier reading, adding further APIs, splitting it
into five steps, and including references to the documentation in
dmaengine.h.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[Fixed the index title to reflect new changes]
Signed-off-by: Vinod Koul <vinod.koul@intel.com>