Add optional dma-coherent property to support coherent PCIe DMA traffic.

Link: https://lore.kernel.org/r/20210222084732.21521-2-bharat.kumar.gogada@xilinx.com


Signed-off-by: Bharat Kumar Gogada <bharat.kumar.gogada@xilinx.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>