mlxsw: spectrum_router: Add support for blackhole nexthops

Add support for blackhole nexthops by programming them to the adjacency
table with a discard action.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>