ath79: Add wifi to WNDR3700, WNDR3700v2 and WNDR3800

Add ath9k wifi capabilities to WNDR3700 family.

* use kmod-owl-loader to load firmware from "art"
* add wifi to DTS
* add wifi LEDs

Avoid using the same MAC for eth0 LAN and wlan0 by
toggling the eth0 MAC into a locally administered MAC.
That is currently done by in user-space by adding a
uci config item into /etc/config/network
(More elegant solution might be setting it already in
preinit phase.)

Known issues:

* wifi firmware file may not get created on the first boot
  after flashing on time to bring wifi normally up. Likely
  the overlay jffs2 is not yet ready for creating the
  firmware file. "wifi up" may still bring wifi up.
  Wifi will work normally at subsequent boots.

* phy0 and phy1 may get assigned mixed, so that phy0 may
  be the 5GHz radio instead of the normal 2.4GHz, and vice
  versa for phy1. Does not happen always, but may happen.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
[fix the wifi unit address in the dts]
Signed-off-by: Mathias Kresin <dev@kresin.me>