net: dsa: bcm_sf2: Add support for port mirroring

We can use b53_mirror_add and b53_mirror_del because the Starfighter 2
is register compatible in that specific case.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>