mlxsw: spectrum_router: Add helpers for nexthop counters

The next patch will add the ability to share nexthop counters among
mlxsw nexthops backed by the same core nexthop. To have a place to store
reference count, the counter should be kept in a dedicated structure. In
this patch, introduce the structure together with the related helpers, sans
the refcount, which comes in the next patch.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/61f23fa4f8c5d7879f68dacd793d8ab7425f33c0.1709901020.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>