net: enetc: report software timestamping via SO_TIMESTAMPING

Let user space properly determine that the enetc driver provides
software timestamps.

Fixes: 4caefbce

 ("enetc: add software timestamping")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Link: https://lore.kernel.org/r/20220324161210.4122281-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>