ramips: add support for DLINK DWR-922-E2

ramips: add support for DLINK DWR-922-E2

Very similar to the DWR-921-C1, except has a telephony/RJ11 port (not
sure if supported, I didn't try), wireless router with QMI 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

The status led has been assigned to the dwr-922-e2:green:signalstrength
(lte signal strength) led. At the end of the boot it is switched off and
is available for lte operation. Works correctly also during sysupgrade
operation.

Installation:
Apply factory image via d-link http web-gui, or via recovery interface:

How to recover/revert to OEM firmware:
1.) Push and hold the reset button and turn on the power. Wait until all
    LEDs start rapidly blinking (~10sec.)
2.) DHCP should give you an IP in the 192.168.123.0/24 subnet, or set
    one manually
3.) Upload original factory image via JBOOT http interface at IP
    192.168.123.254
4.) 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.
5.) You can optionally telnet to 192.168.123.254 before or during the
    upload and it will report the flashing status, memory address etc.
6.) Once web UI and/or telnet says "Success", power cycle the router, or
    type "reboot" into the telnet session.

Signed-off-by: Simon Quigley <squigley@squigley.net>
[squashed commits, word wrap commit message, rename signal strenght led
name to match what is used for the DWR-921-C1 since they share the led
configuration, add label referenced in the aliases node]
Signed-off-by: Mathias Kresin <dev@kresin.me>