ramips: add support for DLINK DWR-921-C3

ramips: add support for DLINK DWR-921-C3

The DWR-921-C3 Wireless Routers with LTE embedded modem is based on the
MT7620N SoC.

Specification:

  * MediaTek MT7620N (580 Mhz)
  * 64 MB of RAM
  * 16 MB of FLASH
  * 802.11bgn radio
  * 5x 10/100 Mbps Ethernet (1 WAN and 4 LAN)
  * 2x external, detachable (LTE) antennas
  * UART header on PCB (57600 8n1)
  * 6x LED (GPIO-controlled)
  * 1x bi-color Signal Strength LED (GPIO-controlled)
  * 2x button
  * JBOOT bootloader

Installation:
Apply factory image via d-link http web-gui.

How to revert to OEM firmware:
1.) Push the reset button and turn on the power. Wait until LED start
    blinking (~10sec.)
2.) Upload original factory image via JBOOT http (IP: 192.168.123.254)
3.) If http doesn't work, it can be done with curl command:
    curl -F FN=@XXXXX.bin http://192.168.123.254/upg
    where XXXXX.bin is name of firmware file.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>