dma: Add Xilinx ZDMA controller driver support

Add driver for Xilinx ZDMA controller. The current implementation
supports memcpy and scatter gather memory transfers.

No support for clock and power management features at this time and
support will be added in subsequent patches.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>