ath79: add support for the TP-LINK WBS510 V2

ath79: add support for the TP-LINK WBS510 V2

This adds support for a popular low-cost 5GHz N based AP

Specifications:
 - SoC: Atheros AR9344
 - RAM: 64MB
 - Storage: 8 MB SPI NOR
 - Wireless: 5GHz 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

Signed-off-by: Andrew Cameron <apcameron@softhome.net>