ath79: add support for TP-Link WBS210 v2

ath79: add support for TP-Link WBS210 v2

TP-Link WBS210 v2 is an outdoor wireless CPE for 2.4 GHz with
two Ethernet ports based on Atheros AR9344

The device is the same as TP-Link CPE220 v2, but with higher TX power (27 dBm
instead of 12 dBm) and two antenna connectors instead of built-in antennas.

Specifications:
 - SoC: Atheros AR9344
 - RAM: 64MB
 - Storage: 8 MB SPI NOR
 - Wireless: 2.4GHz 300 Mbps, 2x RP-SMA connector, 27 dBm TX power
 - Ethernet: 1x 10/100 Mbps with 24V POE IN, 1x 10/100 Mbps

Installation:
Flash factory image through stock firmware WEB UI
or through TFTP
To get to TFTP recovery just hold reset button while powering on for
around 4-5 seconds and release.
Rename factory image to recovery.bin
Stock TFTP server IP: 192.168.0.100
Stock device TFTP adress: 192.168.0.254

The TP-Link WBS devices use the same GPIOs as the CPE devices,
except for the link4 LED. For this one, WBS devices use "2", while
CPE devices use "16". (Tested on WBS210 v2)

Signed-off-by: Bernhard Geier <freifunk@geierb.de>
[added comment about GPIO]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>