Documentation: dmaengine: Add a documentation for the dma controller API

The dmaengine is neither trivial nor properly documented at the moment, which
means a lot of trial and error development, which is not that good for such a
central piece of the system.

Attempt at making such a documentation.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[fixed some minor typos]
Signed-off-by: Vinod Koul <vinod.koul@intel.com>