axivdma: xilinx: Document APIs and structures

Add documentation to APIs and structures as per
kernel-doc.

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
axivdma: xilinx: Use pdev instead of op

Synchronize names with other drivers

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
axivdma: xilinx: Fix checkpatch.pl errors

Fix checkpatch.pl errors

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>