bnxt_en: Add new TX timestamp completion definitions

The new BCM5760X chips will generate this new TX timestamp completion
when a TX packet's timestamp has been taken right before transmission.
The driver logic to retrieve the timestamp will be added in the next
few patches.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>