Documentation: DT: vdma: Add clock support for dmas

This patch updates the binding doc with clock description
for AXI DMA's.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>