mlxsw: Add fields for mlxsw's meta header for adjacency table

This patch adds field for mlxsw's meta header which will be used to
describe the match/action behavior of the adjacency table.

The fields are:
1. Adj_index - The global index of the nexthop group in the adjacency
   table.

2. Adj_hash_index - Local index offset which is based on packets hash
   mod the nexthop group size.

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>