ath79: add support for TP-Link TL-WA850RE v1

ath79: add support for TP-Link TL-WA850RE v1

This ports support for the TL-WA850RE v1 range extender from ar71xx
to ath79.

Specifications:
  Board: AP123 / AR9341 rev. 3
  Flash/RAM: 4/32 MiB
  CPU: 535 MHz
  WiFi: 2.4 GHz b/g/n
  Ethernet: 1 port (100M)

Flashing instructions:
Upload the factory image via the vendor firmware upgrade option.

Recovery:
Note that this device does not provide TFTP via ethernet like many
other TP-Link devices do. You will have to open the case if you
require recovery beyond failsafe.

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