nfp: ethtool: expose transmit SO_TIMESTAMPING capability

NFP always supports software time stamping of tx, now expose
the capability through ethtool ops.

Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Link: https://lore.kernel.org/r/20231129080413.83789-1-louis.peens@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>