mlxsw: spectrum_router: Add gateway indication to nexthop group

The next patch is going to generalize the way in which we store routes.
Instead of attaching a nexthop group only to gatewayed routes, one will
be attached to each route, in a similar way to the way the FIB code
stores its routes.

The above means that any function operating on a nexthop group cannot
assume the group represents only gatewayed nexthops. One such function
is the one that refreshes a nexthop group and updates the adjacency
table following nexthop changes.

For a nexthop group that doesn't represent any gateways this function
would essentially be a NOP, but it would be useful if it did update the
action associated with any route using it. This will allow us to later
consolidate code paths when a nexthop changes following NH_{ADD,DEL}
events.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>