crypto: atmel-aes - add support for Device Tree

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

Initial code by: Nicolas Royer and Eukrea.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>