ath79-Support for TP-Link TL-WA901ND v3/v4/v5

I've performed the following verification steps:

  1. Installed firmware with sysupgrade -n | confirmed working
  2. Connected via SSH to upgraded system. | confirmed working
  3. Checked Ethernet LED operation. | confirmed working
  4. Tested button press on rear of device - WiFi and WiFi LED switches on | confirmed working, see minor issues
  5. WiFi LED, flashes when a device connects | confirmed working
  6. Connect via SSH over WiFi (no ethernet) | confirmed working
  7. Turn security LED (red and grey) on/off by writing to /sys/class/leds/<led>/brightness | confirmed working
  8. Flash back to my previous firmware via ssh | confirmed working

Minor issues - the button on the back of my TL-WA801ND v3 is labelled WPS, not WiFi. I don't care about this, and I know the function can be remapped in software easily.

As far as I'm concerned it looks like things are working fine and ok to merge. Let me know if there's anything more you'd like me to test. Thanks again.