linux-imx

dmaengine: doc: Add ReST style dmaengine document

This removes the index file and adds the index.rst as placeholder
and update driver-api index to add dmaengine. As a consequence
dmaengine documentation will be in driver-api/

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>