fpga: zynq: Use helper functions for zynq dma

Use zynq_dma_xfer_init, zynq_align_dma_buffer,
zynq_dma_transfer helper function performing dma
transfers so that the code can be reused easily for
different cases of dma transfer.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>