mlxsw: spectrum_router: Track nexthop group virtual router membership

For each nexthop group, track in which virtual routers (VRs) the group is
used. This is going to be used by the next patch to perform a more
efficient adjacency index update whenever the group's adjacency index
changes.

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