mlxsw: spectrum_router: Create per-ASIC router operations

There are several differences in the router module between Spectrum-1
and Spectrum-{2,3}. Currently, this is only apparent in the router
interface (RIF) operations that are split between these ASICs.

A subsequent patch is going to introduce another difference between
these ASICs.

Create per-ASIC router operations that will encapsulate all these
differences. For now, these operations are only used to set the per-ASIC
RIF operations in 'mlxsw_sp->router->rif_ops_arr'. Note that this fields
was unused since commit 1f5b2303

 ("mlxsw: spectrum: Set RIF ops per
ASIC type").

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