dmaengine: at_hdmac: extend hardware handshaking interface identification

Peripheral handshaking identification numbers can be bigger than 15, so new
fields have been created in the CFG register.
Add macros to take this modification into account and use them in
at_dma_xlate() function.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>