ramips: add support for ELECOM WRC-X1800GS

ramips: add support for ELECOM WRC-X1800GS

ELECOM WRC-X1800GS is a 2.4/5 GHz band 11ax (Wi-Fi 6) router, based on
MT7621A.

Specification:

- SoC               : MediaTek MT7621A
- RAM               : DDR3 256 MiB
- Flash             : RAW-NAND 128 MiB (Macronix MX30LF1G28AD-TI)
- WLAN              : 2.4/5 GHz 2T2R (MediaTek MT7915D)
- Ethernet          : 5x 10/100/1000 Mbps
  - switch          : MediaTek MT7530 (SoC)
- LEDs/Keys (GPIO)  : 7x/4x
- UART              : pin-header on PCB ("J5")
  - arrangement     : 3.3V, TX, RX, NC, GND from tri-angle marking
  - settings        : 115200n8
- Power             : 12 VDC, 1 A

Flash instruction using initramfs-factory image:

1. Boot WRC-X1800GS normally with "Router" mode
2. Access to "http://192.168.2.1/" and open firmware update page
   ("ファームウェア更新")
3. Select the OpenWrt initramfs-factory image and click apply ("適用")
   button
4. After flashing initramfs-factory image and reboot, upload the
   sysupgrade image and perform sysupgrade with it
5. Wait ~120 seconds to complete flashing

Notes:

- WRC-X1800GS has 2x os images. Those are switched on every firmware
  updating on stock firmware, but dual-boot feature on this device
  cannot be handled on OpenWrt. So the 1st image is always used on
  OpenWrt.
  This is controlled by "bootnum" variable embedded in "persist"
  partition (addr: 0x4).

- WRC-X1800GS has 2x HW revisions. There are some small changes, but the
  same DeviceTree in stock firmware is used for both revisions.
  On this support of WRC-X1800GS, 2x green:wlan-2g-N LEDs are defined
  for each revision and the same default triggers are set.

MAC addresses:

LAN    : 38:97:A4:xx:xx:38 (Factory, 0x1fdfa (hex) / Ubootenv, ethaddr (text))
WAN    : 38:97:A4:xx:xx:3B (Factory, 0x1fdf4 (hex))
2.4 GHz: 38:97:A4:xx:xx:39
5 GHz  : 38:97:A4:xx:xx:3A

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>