generic: ar8216: add port mirroring

Tested on Buffalo WZR-600DHP with ar8316 switch. Commands used to mirror both
RX and TX traffic from LAN port 1 to LAN port 4:

$ swconfig dev switch0 set enable_mirror_rx 1
$ swconfig dev switch0 set enable_mirror_tx 1
$ swconfig dev switch0 set mirror_monitor_port 4
$ swconfig dev switch0 set mirror_source_port 1

Signed-off-by: Colin Leitner <colin.leitner@googlemail.com>
Tested-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36713 3c298f89-4303-0410-b956-a3cf2f4a3e73