ath79: add support for TP-Link TL-WA901ND v3

ath79: add support for TP-Link TL-WA901ND v3

This ports support for the TL-WA901ND v3 from ar71xx to ath79.
Most of the hardware is shared with the TL-WA850/860RE v1 range
extenders. It completes the TL-WA901ND series in ath79.

Specifications:
  Board: AP123 / AR9341
  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.

This has not been tested on device, but port from ar71xx is
straightforward and the device will be disabled by default anyway.

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