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: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>