mediatek: add support for Cudy WR3000U v1

mediatek: add support for Cudy WR3000U v1

Hardware:
 - SoC: MediaTek MT7981B
 - CPU: 2x 1.3 GHz Cortex-A53
 - Flash: 256 MiB ESMT F50L2G41XA
 - RAM: DDR3, 512 MiB
 - WLAN: 2.4 GHz, 5 GHz (MediaTek MT7976CN, 802.11ax)
 - 1x WAN MT7531 (JXD 2531Z) 10/100/1000 Mbps
 - 4x LAN 2x MT7530 (JXD 2529S) 10/100/1000 Mbps
 - USB 3.0 port
 - Buttons: Reset, WPS
 - 8x LEDs: 2x Red, 6x Blue
 - Serial console: no need to solder. 115200 8n1
 - Power: 12 VDC, 1.5 A

The DTS inherits WBR3000UAX because WR3000U is almost the same.
The only difference is size of NAND and factory UBI length.

+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| WAN     | D4:0D:AB:xx:xx:x1 | label+1   |
| LAN     | D4:0D:AB:xx:xx:x0 | label     |
| WLAN 2g | D4:0D:AB:xx:xx:x0 | label     |
| WLAN 5g | D6:0D:AB:xx:xx:x1 | label+1   |
+---------+-------------------+-----------+

There are 2 ways to install OpenWrt:
- via an intermediate RSA-signed vendor's image or
- via UART && TFTPd

Migration to OpenWrt via OEM firmware:
- Download the migration image from
  https://drive.google.com/drive/folders/1BKVarlwlNxf7uJUtRhuMGUqeCa5KpMnj
  (The folder is Cudy's official "Cudy Intermediary OpenWrt Firmware" resource)
- Upload the migration image via OEM web interface
- After flashing, OpenWrt is accessible via http://192.168.1.1
- Flash the official OpenWrt image

Install using UART & TFTP:
- Connect to UART.
- Connect to LAN and set your IP to 192.168.1.88/24.
- Configure a TFTP server to serve
  openwrt-mediatek-filogic-cudy_wr3000u-v1-initramfs-kernel.bin file
  and run the TFTP server.
- Press reset and power-on the router. Hold the reset button for 5 seconds
- TFTP error: 'File not found' appears and MTK console becomes available
- Enter the following commands in the console:
  setenv bootfile openwrt-mediatek-filogic-cudy_wr3000u-v1-initramfs-kernel.bin
  tftpboot
  bootm
- After booting to initramfs, via http://192.168.1.1 flash
  openwrt-mediatek-filogic-cudy_wr3000u-v1-squashfs-sysupgrade.bin
- After flashing, OpenWrt is accessible via http://192.168.1.1

If you fail to flash the device,
you can use TFTP to flash back to the original firmware:
- Download the original firmware from
  https://www.cudy.com/pages/download-center/wr3000u-256mb-1-0
- Connect to LAN and set your IP to 192.168.1.88/24.
  Configure a TFTP server and an recovery.bin firmware file
- With the router off, press the RESET button.
  While the router is turning on,
  the button should continue to be pressed for at least 5 seconds
- After 1-2 minutes the original firmware is available on http://192.168.10.1

Signed-off-by: Dmitry Mostovoy <stavultras@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24447
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>