dmaengine: at_hdmac: add device tree support

Add device tree probe support for atmel at_hdmac DMA driver.
Bindings are added to specify DMA controller configuration.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>