ath79: provide label MAC address

ath79: provide label MAC address

This patch adds the label MAC address for several devices in
ath79.

Some devices require setting the MAC address in 02_network:

For the following devices, the netif device can be linked in
device tree, but the MAC address cannot be read:
- alfa-network,ap121f
- avm,fritz300e
- ubnt-xm devices

For the following devices, label MAC address is tied to lan or
wan, so no node to link to exists in device tree:
- adtran,bsap1800-v2
- adtran,bsap1840
- dlink,dir-842-c1/-c2/-c3
- engenius,ecb1750
- iodata,etg3-r
- iodata,wn-ac1167dgr
- iodata,wn-ac1600dgr
- iodata,wn-ac1600dgr2
- iodata,wn-ag300dgr
- nec,wg800hp
- nec,wg1200cr
- trendnet,tew-823dru

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>