ar8216: add swconfig attribute to display ARL table on AR8327/AR8337

ar8216: add swconfig attribute to display ARL table on AR8327/AR8337

Add global read-only swconfig attribute "arl_table" to display the
address resolution table.
So far the chip-specific part is implemented for AR8327/AR8337 only
as I don't have the datasheets for the other AR8XXX chips.

Successfully tested on TL-WDR4300 (AR8327rev2)
and TL-WDR4900 (AR8327rev4).

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44104 3c298f89-4303-0410-b956-a3cf2f4a3e73