mlxsw: spectrum_span: Add APIs to enable / disable global mirroring triggers

While the binding of global mirroring triggers to a SPAN agent is
global, packets are only mirrored if they belong to a port and TC on
which the trigger was enabled. This allows, for example, to mirror
packets that were tail-dropped on a specific netdev.

Implement the operations that allow to enable / disable a global
mirroring trigger on a specific port and TC.

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