Add support for Device Tree and use of the DMA DT API to
get the channels if needed.
Documentation is added for these DT nodes.

Initial code by: Nicolas Royer and Eukrea.
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: NHerbert Xu <herbert@gondor.apana.org.au>