mlxsw: spectrum_router: Add support for resilient nexthop groups

Parse the configuration of resilient nexthop groups to existing mlxsw
data structures. Unlike non-resilient groups, nexthops without a valid
MAC or router interface (RIF) are programmed with a trap action instead
of not being programmed at all.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>