dpaa_eth: add struct for software backpointers

We maintain an skb backpointer in the software annotations area of Tx
frames. Introduce a structure for explicit handling.

Acked-by: Madalin Bucur <madalin.bucur@oss.nxp.com>
Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>