ath79: add support for TP-Link TL-WA850RE v2

ath79: 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

Stock firmware uses label MAC address for WiFi and same with local
bit set for ethernet. Since this is difficult to reproduce with
the toolset of OpenWrt, we just keep both ethernet and WiFi to
the same address here.

This is the first tiny device with tplink-safeloader in ath79.
Firmware partition is only 3648k and thus even smaller than for
the tplink-4m(lzma) devices.

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