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>

Documentation/driver-api/index.rst

@@ -46,6 +46,7 @@ available subsections can be seen below.
    pinctl
    gpio
    misc_devices
+   dmaengine/index
 
 .. only::  subproject and html