net/mlx5: support software TX timestamp

net/mlx5: support software TX timestamp

Having a software timestamp (along with existing hardware one) is
useful to trace how the packets flow through the stack.
mlx5e_tx_skb_update_hwts_flags is called from tx paths
to setup HW timestamp; extend it to add software one as well.

Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Stanislav Fomichev <stfomichev@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20250508235109.585096-1-stfomichev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>