ath79: add support for Hak5 Packet Squirrel

ath79: add support for Hak5 Packet Squirrel

Hak5 Packet Squirrel is a pocket-sized device dedicated for pentesters
(MITM attacks). This device is based on Atheros AR9331 but it lacks
WiFi. Support for it was first introduced in 950b278c81 (ar71xx).

Specifications:

- Atheros AR9331
- 400/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR2)
- 16 MB of flash (SPI NOR)
- 2x RJ45 10/100 Mbps Ethernet (AR9331)
- 1x USB 2.0
- 1x RGB LED, 1x button, 1x 4-way mechanical switch
- 1x Micro USB Type-B for main power input

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>