dt-bindings: dmaengine: xilinx_dma: Add VDMA vertical flip property

The AXI VDMA core supports Vertical flip in S2MM path when Enable
Vertical Flip (Advanced tab) is selected. To allow vertical flip
programming define an optional 'xlnx,enable-vert-flip' channel
child node property.

Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Kedareswara rao Appana <appanad@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>