devlink: Add Ethernet header for dpipe

This will be used by the IPv4 host table which will be introduced in the
following patches. This header is global and can be reused by many
drivers.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>