gianfar: Add hardware RX timestamping support

The device is configured to insert hardware timestamps into all
received packets. The RX timestamps are extracted from the padding
alingment bytes during the clean_rx_ring operation and copied into the
skb_shared_hwtstamps struct of the skb. This extraction only happens if
the rx_filter was set to something else than HWTSTAMP_FILTER_NONE with
the SIOCSHWTSTAMP ioctl command.

Hardware timestamping is only supported for eTSEC devices. To indicate
device support the new FSL_GIANFAR_DEV_HAS_TIMER flag was introduced.

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