ath79: add support for TP-Link CPE220 v2

ath79: add support for TP-Link CPE220 v2

This seems to be identical to CPE210 v1 despite having removable
antennas.

Specifications:

    * SoC: Qualcomm Atheros AR9344 (560 MHz)
    * RAM: 64MB
    * Storage: 8 MB
    * Wireless: 2.4GHz N based built into SoC 2x2
    * Ethernet: 2x 100/10 Mbps, integrated into SoC, 24V POE IN

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 address:192.168.0.254

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>