gianfar: Add hardware TX timestamping support

If a packet has the skb_shared_tx->hardware flag set the device is
instructed to generate a TX timestamp and write it back to memory after
the frame is transmitted. During the clean_tx_ring operation the
timestamp will be extracted and copied into the skb_shared_hwtstamps
struct of the skb.

TX timestamping is enabled by setting the tx_type to something else
than HWTSTAMP_TX_OFF with the SIOCSHWTSTAMP ioctl command. It is only
supported by eTSEC devices.

Signed-off-by: Manfred Rudigier <manfred.rudigier@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>