ath79: add support for TP-Link TL-WR940N v6

ath79: add support for TP-Link TL-WR940N v6

The TL-WR940N v6 is similar to v3/v4, it just has different
LEDs and MAC address assignment.

Specification:
- 750 MHz CPU
- 32 MB of RAM
- 4 MB of FLASH
- 2.4 GHz WiFi
- 4x 10/100 Mbps Ethernet

The use of LEDs is based on ar71xx, so blue LED is used for WAN
and orange LED for diag (boot/failsafe/etc.).

Flash instruction (WebUI):
Download *-factory.bin image and upload it via the firmwary upgrade
function of the stock firmware WebUI.

Flash instruction (TFTP):
1. Set PC to fixed ip address 192.168.0.66
2. Download *-factory.bin image and rename it to
   wr940nv6_tp_recovery.bin
3. Start a tftp server with the image file in its root directory
4. Turn off the router
5. Press and hold Reset button
6. Turn on router with the reset button pressed and wait ~15 seconds
7. Release the reset button and after a short time
the firmware should be transferred from the tftp server
8. Wait ~30 second to complete recovery.

Thanks to Manuel Kock for reviewing and testing this patch.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Manuel Kock <github.web@manu.li>
(backported from commit fbd00bb8d406a0ace7ea9c49a79cbad2418689a5)