ath79: add support for Buffalo WZR-600DHP

ath79: add support for Buffalo WZR-600DHP

The hardware of this device seems to be identical to WZR-HP-AG300H.
It was already implemented as a clone in ar71xx.

Specification:
- 680 MHz CPU (Qualcomm Atheros AR7161)
- 128 MiB RAM
- 32 MiB Flash
- WiFi 5 GHz a/n
- WiFi 2.4 GHz b/g/n
- 5x 1000Base-T Ethernet
- 1x USB 2.0

Installation of OpenWRT from vendor firmware:
- Connect to the Web-interface at http://192.168.11.1
- Go to “Administration” → “Firmware Upgrade”
- Upload the OpenWrt factory image

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