mlxsw: spectrum_router: Add helpers for nexthop access

This is done as a preparation before introducing the ability to dump the
adjacency table via dpipe, and to count the table size. The current table
implementation avoids tunnel entries, thus a helper for checking if
the nexthop group contains tunnel entries is also provided. The mlxsw's
nexthop representative struct stays private to the router module.

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>