veth: add software timestamping

Provide a software TX timestamp as well as the ethtool query interface
and report the software timestamp capabilities.

Tested with "ethtool -T" and two linuxptp instances each bound to a
tunnel endpoint.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>