dt: bindings: dma: xilinx: dpdma: DT bindings for Xilinx DPDMA

The ZynqMP includes the DisplayPort subsystem with its own DMA engine
called DPDMA. The DPDMA IP comes with 6 individual channels
(4 for display, 2 for audio). This documentation describes DT bindings
of DPDMA.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>