ar71xx: add software transmit timestamp support

Add software transmit timestamp and ethtool (-T) timestamp support

Signed-off-by: Stephen Walker <stephendwalker+github@gmail.com>

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_ethtool.c

@@ -127,4 +127,5 @@ struct ethtool_ops ag71xx_ethtool_ops = {
 	.get_ringparam	= ag71xx_ethtool_get_ringparam,
 	.set_ringparam	= ag71xx_ethtool_set_ringparam,
 	.get_link	= ethtool_op_get_link,
+	.get_ts_info	= ethtool_op_get_ts_info,
 };

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -819,6 +819,8 @@ static netdev_tx_t ag71xx_hard_start_xmit(struct sk_buff *skb,
 
 	netdev_sent_queue(dev, skb->len);
 
+	skb_tx_timestamp(skb);
+
 	desc->ctrl &= ~DESC_EMPTY;
 	ring->curr += n;