mlxsw: spectrum_dpipe: Add initial support for the router adjacency table

Add initial support for router adjacency table. The table does lookup
based on the nexthop-group index and the local nexthop offset. After
locating the nexthop entry it sets the destination MAC address and the
egress RIF.

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>