ar71xx: add support for GL.iNet GL-USB150

ar71xx: add support for GL.iNet GL-USB150

GL.iNet GL-USB150 is an USB dongle WiFi router, based on Atheros AR9331.

Specification:

- 400/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- Realtek RTL8152B USB to Ethernet bridge (connected with AR9331 PHY4)
- 1T1R 2.4 GHz
- 2x LED, 1x button
- UART header on PCB

Flash instruction:

Vendor firmware is based on OpenWrt CC. GUI or sysupgrade can be used
to flash LEDE firmware.

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