mlxsw: spectrum_span: Add per-ASIC SPAN agent operations

The various SPAN agent types differ in their mirror targets (i.e.,
physical port netdev vs. VLAN netdev) and the encapsulation headers that
they need to encapsulate the mirrored packets with.

The Spectrum-2 and Spectrum-3 ASICs support a SPAN agent type that is
able to mirror towards the CPU, whereas the Spectrum-1 ASIC does not.

Prepare for the addition of this new SPAN agent type by splitting the
SPAN agent operations to be per-ASIC.

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