ar8216: add reading ARL table for AR8216/AR8236/AR8316

Adds the chip-specific part of reading ARL table for AR8216/AR8236/AR8316.

It's based on the AR8236 datasheet and compile-tested only as I couldn't
find datasheets for AR8216/AR8316 and don't own devices with these chips.

The existing ar8216_atu_flush implementation was used for all three
chip types, therefore I guess they share a common ATU register layout.

More testing would be appreciated.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46379 3c298f89-4303-0410-b956-a3cf2f4a3e73