ar71xx: add support for ALFA Network AP121F

ar71xx: add support for ALFA Network AP121F

ALFA Network AP121F is a pocket-size router dedicated for VPN/TOR users.
Device is based on Atheros AR9331 WiSoC and is running a custom version
(updated from OpenWrt CC to LEDE 17.01 release) of NetAidKit firmware.

Specification:

- 400/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM (DDR1)
- 16 MB of FLASH (SPI NOR)
- 1x 10/100 Mbps Ethernet
- 1T1R 2.4 GHz
- 1x microSD (optional, on separate PCB)
- 3x LED, 1x button, 1x switch
- UART header on PCB

Flash instruction (under U-Boot web recovery mode):

1. Configure PC with static IP 192.168.1.2/24.
2. Connect PC with RJ45 port, press the reset button, power up device,
   wait for first blink of all LEDs (indicates network setup), then keep
   button for 3 following blinks and release it.
3. Open 192.168.1.1 address in your browser and upload sysupgrade image.

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