ramips: add support for Fon FON2601

ramips: add support for Fon FON2601

FON2601 is a wireless router.

Specification:
- SoC: Mediatek MT7620A (580MHz)
- RAM: 128 MiB
- ROM: 16 MiB SPI Flash
- Wireless:
   for 11b/g/n (upto 300 Mbps):  MT7620A built-in WMAC
   for 11a/n/ac (upto 867 Mbps): MT7662E
- Ethernet LAN: 1 port, upto 100 Mbps
- Ethernet WAN: 1 port, upto 1000 Mbps
- USB: 1 port (USB 2.0 host)
- LEDs: 4 (all can be controlled by SoC's GPIO)
- buttons: 1 (Displayed as "WPS" on enclosure)
- serial port: 57600n8
 pins: Vcc(3.3V), Rx, Tx, GND
(left to right, viewed from outside of board)

Installation (only available via UART):
  1. download sysupgrade binary image by wget command
  2. write sysupgrade binary image to Flash
     command is:
       mtd write sysupgrade.bin firmware
  3. reboot

Important Notice:
  Only one button is displayed as "WPS" on enclosure.
  However, it is configured as "reset" (factory resetting feature).

Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>
[removed unrelated openwrt-keyring revert, missing -Wall for uimage_padhdr]
Signed-off-by: Petr Štetiar <ynezz@true.cz>