ramips: add support for ELECOM WMC-X1800GST

ramips: add support for ELECOM WMC-X1800GST

ELECOM WMC-X1800GST is a 2.4/5 GHz band 11ax (Wi-Fi 6) mesh router,
based on MT7621A

Specification:

- SoC             : MediaTek MT7621A
- RAM             : DDR3 512 MiB (Nanya NT5CC256M16ER-EK)
- Flash           : RAW-NAND 128 MiB (Winbond W29N01HVSINF)
- WLAN            : 2.4/5 GHz 2T2R (MediaTek MT7915D + MT7975D)
- Ethernet        : 3x 10/100/1000 Mbps
  - switch        : MediaTek MT7530 (SoC)
- LEDs/Keys (GPIO): 9x/5x
- UART            : through-hole on PCB ("J4")
  - arrangement   : 3.3V, GND, TX, RX from tri-angle marking
  - settings      : 115200n8
- Power           : 12 VDC, 1 A (Max. 11.5 W)

Flash instruction using initramfs-factory image

1. Boot WMC-X1800GST 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. On initramfs image, download sysupgrade image to the device and
   perform sysupgrade with that image
5. Wait ~120 seconds to complete flashing

Notes:

- The "firmware" partition on the stock image is only 0xF00000 (15 MiB)
  and it's too small for the current OpenWrt firmware with UBI format.
  So use the unused area at the end of NAND flash for rootfs (UBI).

MAC addresses:

LAN    : 04:AB:18:xx:xx:BF (Factory, 0x3fff4 (hex))
WAN    : 04:AB:18:xx:xx:C0 (Factory, 0x3fffa (hex))
2.4 GHz: 04:AB:18:xx:xx:C1 (Factory,     0x4 (hex))
5 GHz  : 04:AB:18:xx:xx:C2

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16384
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>