ath79: add support for ALFA Network AP121FE

ath79: add support for ALFA Network AP121FE

The AP121FE is a slightly modified version of already supported AP121F
model (added to ar71xx in 0c6165d21a and to ath79 in 334bbc5198).

The differences in compare to AP121F:

- no micro SD card reader
- USB data lines are included in Type-A plug
- USB bus switched to device/peripheral mode (permanently, in bootstrap)

Other than that, specifications are the same:

- Atheros AR9331
- 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 Wi-Fi, up to 15 dBm
- 1x IPEX/U.FL connector, internal PCB antenna
- 3x LED, 1x button, 1x switch
- 4-pin UART header on PCB (2 mm pitch)
- USB 2.0 Type-A plug (power and data)

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>