ath79: add support for Hak5 LAN Turtle

ath79: add support for Hak5 LAN Turtle

Hak5 LAN Turtle is an "USB Ethernet Adapter" shaped device dedicated for
sysadmins and pentesters. This device is based on Atheros AR9331 but it
lacks WiFi. Support for it was first introduced in 950b278c81 (ar71xx).

Two different versions of this device exist and it's up to the user to
install required drivers (generic image supports only common features):

- LAN Turtle 3G with Quectel UG96 3G modem
- LAN Turtle SD with microSD card reader (Alcorlink AU6435R)

Specifications:

- Atheros AR9331
- 400/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 16 MB of flash (SPI NOR)
- 1x RJ45 10/100 Mbps Ethernet (AR9331)
- 1x USB 2.0 to 10/100 Ethernet bridge (Realtek RTL8152B)
- 2x LED (power, system), 1x button (inside, on the PCB)
- USB 2.0 Type-A plug for power and RTL8152B

Flash instruction:

You can use sysupgrade image directly in vendor firmware which is based
on OpenWrt/LEDE.

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