ath79: add support for TP-Link WDR3500 v1

ath79: add support for TP-Link WDR3500 v1

Hardware:
SoC:      AR9344
CPU:      560 MHz
Flash:    8 MiB
RAM:      128 MiB
WiFi:     Atheros AR9340 2.4GHz 802.11bgn
          Atheros AR9300 5GHz 802.11an
Ethernet: AR934X built-in switch, WAN on separate physical interface
USB:      1x 2.0

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
   wdr3500v1_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.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[removed stray newline]
Signed-off-by: David Bauer <mail@david-bauer.net>