dma: xilinx: Code cleanup in the driver

This patch includes
- Remove unnecessary header files
- Rename the driver API names to sync with mainline driver api names
- Document the driver API's
- Remove unnecessary code.

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