mlxsw: spectrum_span: Add APIs to bind / unbind a SPAN agent

Currently, a SPAN agent can only be bound to a per-port trigger where
the trigger is either an incoming packet (INGRESS) or an outgoing packet
(EGRESS) to / from the port.

A follow-up patch set will introduce the concept of global triggers and
per-{port, TC} enablement. With global triggers, the trigger entry is
only keyed by a trigger and not by a port and a trigger. The trigger can
be, for example, a packet that was early dropped.

While the binding between the SPAN agent and the trigger is performed
only once, the trigger entry needs to be reference counted, as the
trigger can be enabled on multiple ports.

Add APIs to bind / unbind a SPAN agent to a trigger and reference count
the trigger entry in preparation for global triggers.

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