net: dsa: qca8k: add support for mirror mode

The switch supports mirror mode. Only one port can set as mirror port and
every other port can set to both ingress and egress mode. The mirror
port is disabled and reverted to normal operation once every port is
removed from sending packet to it.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>