ar71xx: add support for TRENDnet TEW-823DRU

Add support for TRENDnet TEW-823DRU:
- cpu: QCA9558 @720MHz
- flash: 16MB
- ram: 256MB
- 4+1 RJ45 100/1000Mbps
- wifi:  QCA9558 (bgn) and QCA9880-BR4A (ac)
- 1x USB 2.0

Signed-off-by: Cezary Jackiewicz <cezary.jackiewicz@gmail.com>

SVN-Revision: 47927