mlxsw: spectrum_router: Encode adjacency group size ranges in an array

The device supports a fixed set of adjacency group sizes. Encode these
sizes in an array, so that the next patch will be able to split it
between Spectrum-1 and Spectrum-{2,3}, which support different size
ranges.

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