dwc_eth_qos: Add Synopsys DWC Ethernet QoS bindings

Add device tree binding documentation for the Synopsys DWC Ethernet
QoS driver supporting revision 4.10a of the hardware IP.

Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>