dmaengine: at_hdmac: simplify device selection from platform data or DT

Using a configuration structure simplify the finding of SoC
dependent parameters. Both platform data and device tree ids are
using these structures.
This will separate data from code and remove the need for an enum.

Idea from Grant Likely.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>