PCI: xilinx-nwl: Add optional "dma-coherent" property

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>

Documentation/devicetree/bindings/pci/xilinx-nwl-pcie.txt

@@ -33,6 +33,8 @@ Required properties:
 	- #address-cells: specifies the number of cells needed to encode an
 		address. The value must be 0.
 
+Optional properties:
+- dma-coherent: present if DMA operations are coherent
 
 Example:
 ++++++++