ar71xx: add support for Zbtlink ZBT-WE1526

Zbtlink ZBT-WE1526 is based on Qualcomm Atheros QCA9531 v2.
Short specification:

- 650/400/200 MHz (CPU/DDR/AHB)
- 5x 10/100 Mbps Ethernet
- 1x USB 2.0
- 128 MB of RAM (DDR2)
- 16 MB of FLASH
- 2T2R 2.4 GHz, up to 22 dBm
- two external, non-detachable antennas
- 8x LED, 1x button
- UART header (pinout: VCC, RX, TX, GND)

Flash instruction:

Use sysupgrade in vendor firmare which is based on OpenWrt.

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