amd-xgbe: Base AXI DMA cache settings on device tree

The default cache operations for ARM64 were changed during 3.15.
To use coherent operations a "dma-coherent" device tree property
is required.  If that property is not present in the device tree
node then the non-coherent operations are assigned for the device.

Add support to the amd-xgbe driver to assign the AXI DMA cache settings
based on whether the "dma-coherent" property is present in the device
node.  If present, use settings that work with the caches.  If not
present, use settings that do not look at the caches.

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