mlxsw: spectrum_router: Parametrize RIF allocation size

Currently, each router interface (RIF) consumes one entry in the RIFs
table. This is going to change in subsequent patches where some RIFs
will consume two table entries.

Prepare for this change by parametrizing the RIF allocation size. For
now, always pass '1'.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>