mlxsw: spectrum_router: Add support for custom multipath hash policy

When this policy is set, only enable the packet fields that were enabled
by user space for multipath hash computation.

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