ramips: add support for ALFA Network AWUSFREE1

ramips: add support for ALFA Network AWUSFREE1

ALFA Network AWUSFREE1 is an USB Wi-Fi N300 adapter based on MT7628.

Specification:

- MT7628AN (580 MHz)
- 64 MB of RAM (DDR2)
- 8 MB of FLASH (SPI NOR)
- 2T2R 2.4 GHz (MT7628) with external FEM (RFFM4203)
- 2x detachable antennas (RP-SMA)
- ASIX AX88772 USB to Ethernet bridge (connected with MT7628 PHY0)
- 4x LED (2 driven by GPIO)
- 1x button (reset)
- 1x mini USB for host and main power input
- UART header on PCB

Flash instruction:

You can use sysupgrade image directly in vendor firmware which is based
on LEDE/OpenWrt. Alternatively, you can use web recovery mode in U-Boot:

1. Power device with reset button pressed and release it after ~5 sec.
2. Setup static IP 192.168.1.2/4 on your PC.
3. Go to 192.168.1.1 in browser and upload "sysupgrade" image.

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