amd-xgbe: Add dma-coherent to device bindings documentation

An earlier patch added support for the "dma-coherent" device property.
This patch adds this optional property to the amd-xgbe device bindings
documentation.
Signed-off-by: NTom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/amd-xgbe.txt

@@ -18,6 +18,7 @@ Required properties:
 Optional properties:
 - mac-address: mac address to be assigned to the device. Can be overridden
   by UEFI.
+- dma-coherent: Present if dma operations are coherent
 
 Example:
 	xgbe@e0700000 {