net: dsa: sja1105: Add support for port mirroring

Amazingly, of all features, this does not require a switch reset.

Tested with:

tc qdisc add dev swp2 clsact
tc filter add dev swp2 ingress matchall skip_sw \
	action mirred egress mirror dev swp3
tc filter show dev swp2 ingress
tc filter del dev swp2 ingress pref 49152

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>