ethtool: add timestamping related string sets

Add three string sets related to timestamping information:

  ETH_SS_SOF_TIMESTAMPING: SOF_TIMESTAMPING_* flags
  ETH_SS_TS_TX_TYPES:      timestamping Tx types
  ETH_SS_TS_RX_FILTERS:    timestamping Rx filters

These will be used for TIMESTAMP_GET request.

v2: avoid compiler warning ("enumeration value not handled in switch")
    in net_hwtstamp_validate()

v3: omit dash in Tx type names ("one-step-*" -> "onestep-*"), suggested by
    Richard Cochran

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>