ath79: add support for GL.iNet GL-USB150

ath79: add support for GL.iNet GL-USB150

Add support for the ar71xx supported GL.iNet GL-USB150 to ath79.

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 software is based on openwrt so you can flash the sysupgrade
image via the vendor GUI or using command line sysupgrade utility.
Make sure to not save configuration over reflash as uci settings
differ between versions.

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>