ramips: add support for I-O DATA WN-DEAX1800GR

ramips: add support for I-O DATA WN-DEAX1800GR

I-O DATA WN-DEAX1800GR is a 2.4/5 GHz band 11ax (Wi-Fi 6) router, based
on MT7621A.

Specification:

- SoC         : MediaTek MT7621A
- RAM         : DDR3 256 MiB (Nanya NT5CC128M16JR-EK)
- Flash       : RAW NAND 128 MiB (Winbond W29N01HVSINF)
- WLAN        : 2.4/5 GHz (MediaTek MT7915)
- Ethernet    : 5x 10/100/1000 Mbps
  - Switch    : MT7530 (SoC)
- LEDs/Keys   : 6x/3x
- UART        : through-hole on PCB (J2)
  - assignment: 3.3V, GND, TX, RX from "1" marking
  - settings  : 115200n8
- Power       : 12 VDC, 1 A

Flash instruction using initramfs-factory image:

1. Boot WN-DEAX1800GR normally
2. Access to "http://192.168.0.1/" and open firmware update page
   ("ファームウェア")
3. Select the OpenWrt initramfs-factory.bin image and click update
   ("更新") button to perform firmware update
4. On the initramfs image, perform sysupgrade with the
   squashfs-sysupgrade.bin image
5. Wait ~120 seconds to complete flashing

Note:

- This device has 2x OS images on the flash storage. In this support,
  the first one will be used.

Warning:

- Do not use "saveenv" command on U-Boot CLI.
  This device has wrong u-boot-env data. The actual length of individual
  env data installed to the device is 0x1000 (4 KiB), but installed
  U-Boot requires 0x20000 (128 KiB). So U-Boot determines the data is
  invalid. Then, if you perform saving environment data with saveenv on
  U-Boot CLI, installed env data will be overwritten with too few
  default values without individual values (SSID, password, MAC
  addresses, etc...).

MAC addresses:

LAN    : 50:41:B9:xx:xx:F4 (Config, ethaddr (text))
WAN    : 50:41:B9:xx:xx:F6 (Config, wanaddr (text))
2.4 GHz: 50:41:B9:xx:xx:F4 (Config, rmac (text) / Factory, 0x4 (hex))
5 GHz  : 50:41:B9:xx:xx:F5 (none)

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