dt-bindings: net: snps,dwmac: Tx coe unsupported

Add dt-bindings for coe-unsupported property per tx queue. Some DWMAC
IPs support tx checksum offloading(coe) only for a few tx queues.

DW xGMAC IP can be synthesized such that it can support tx coe only
for a few initial tx queues. Also as Serge pointed out, for the DW
QoS IP tx coe can be individually configured for each tx queue. This
property is added to have sw fallback for checksum calculation if a
tx queue doesn't support tx coe.

Signed-off-by: Rohan G Thomas <rohan.g.thomas@intel.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>