mlxsw: reg: Add egress FID field to RITR register

RITR configures the router interface table. As preparation for unified
bridge model, add egress FID field to RITR.

After routing, a packet has to perform a layer-2 lookup using the
destination MAC it got from the routing and a FID.
In the new model, the egress FID is configured by RITR for both sub-port
and FID RIFs.

Add 'efid' field to sub-port router interface and update FID router
interface related comment.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>