ar71xx: add support for TP-Link TL-WA850RE v2

TP-Link TL-WA850RE v2 is a wall-plug N300 Wi-Fi range extender,
based on Qualcomm/Atheros QCA9533 v2.

Short specification:

- 550/391/195 MHz (CPU/DDR/AHB)
- 1x 10/100 Mbps Ethernet
- 32 MB of RAM (DDR1)
- 4 MB of FLASH
- 2T2R 2.4 GHz
- 2x internal antennas (embedded on PCB)
- 9x LED (all can be turned off with GPIO15), 2x button
- UART (J3) header on PCB

Flash instruction: use "factory" image directly in vendor GUI.

Warning: this device does not include any kind of recovery mechanism
in the bootloader and disassembling process is not trivial.

You can access vendor firmware over serial line using:
- login: root
- password: sohoadmin

Image was tested only in US version of the device, but should work
also with the same device version sold in other countries.

Available FLASH space, with LEDE trunk, is only 240 KB.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>