ar71xx: add support for TP-Link TL-WA855RE v1

ar71xx: add support for TP-Link TL-WA855RE v1

TP-Link TL-WA855RE v1 is a wall-plug N300 Wi-Fi range extender,
based on Qualcomm/Atheros QCA9533 v2.

Short specification:

- 550/397/198 MHz (CPU/DDR/AHB)
- 1x 10/100 Mbps Ethernet
- 32 MB of RAM (DDR1)
- 4 MB of FLASH
- 2T2R 2.4 GHz
- 2x external antennas
- 2x LED (green and orange in the same package), 2x button
- UART: TP5(TX) and TP4(RX) test points 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 EU version of the device, but should work
also with the same device version sold in other countries.

Signed-off-by: Federico Cappon <dududede371@gmail.com>