dma: xilinx: Support queuing multiple transactions

Create a xilinx_dma_tx_descriptor structure and use it model DMA
transactions. A transaction can contain several segments, and multiple
transactions can be prepared and submitted without waiting for the
previous transactions to complete.

Replace the active descriptors list with an active descriptor pointer,
and add a new complete descriptors list to store complete descriptors
that need to be handed back to the client.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>