ramips: add support for JCG Q20

JCG Q20 is an AX 1800M router.

Hardware specs:
  SoC: MediaTek MT7621AT
  Flash: Winbond W29N01HV 128 MiB
  RAM: Winbond W632GU6NB-11 256 MiB
  WiFi: MT7915 2.4/5 GHz 2T2R
  Ethernet: 10/100/1000 Mbps x3
  LED: Status (red / blue)
  Button: Reset, WPS
  Power: DC 12V,1A

Flash instructions:
  Upload factory.bin in stock firmware's upgrade page,
  do not preserve settings.

MAC addresses map:
  0x00004 *:3e wlan2g/wlan5g
  0x3fff4 *:3c lan/label
  0x3fffa *:3c wan

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>