ramips: add support for ELECOM WRC-1900GST

ramips: add support for ELECOM WRC-1900GST

ELECOM WRC-1900GST is a wireless router, based on Mediatek MT7621A.
This is almost same as WRC-2533GST except wireless specs.

Specifications:

  - SoC : MT7621A (four logical CPU cores)
  - RAM : 128MiB
  - ROM : 16MiB of SPI NOR-FLASH
  - wireless :
          5GHz : 3T3R up to 1300Mbps/11ac with MT7615
          2.4GHz : 3T3R up to 600Mbps/11n with MT7615
  - Ethernet : 5 ports, all ports is capable of 1000base-T
  - Ether switch : MT7530 (MT7621A built-in)
  - LEDs : 4 LEDs
  - buttons : 2 buttons and 1 slide-switch
  - UART : header is on PCB, 57600bps

Flash instruction using factory image:

 1. Connect the computer to the LAN port of WRC-1900GST
 2. Connect power cable to WRC-1900GST and turn on it
 3. Access to "https://192.168.2.1/" and open firmware update
    page ("ファームウェア更新")
 4. Select the OpenWrt factory image and click apply ("適用")
    button
 5. Wait ~150 seconds to complete flashing

Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>
(backported from ad10e71becfb36d36c81c5b0e898c4ea7912d778)