mediatek: add support for Cudy TR3000 v1

mediatek: add support for Cudy TR3000 v1

Hardware:
 - SoC: MediaTek MT7981B
 - CPU: 2x 1.3 GHz Cortex-A53
 - Flash: 128 MiB SPI NAND
 - RAM: 512 MiB
 - WLAN: 2.4 GHz, 5 GHz (MediaTek MT7976CN, 802.11ax)
 - Ethernet: 1x 10/100/1000/2500 Mbps RTL8221B WAN, 1x10/100/1000 Mbps MT7981 LAN
 - USB 3.0 port
 - Buttons: 1 Reset button, 1 slider button
 - LEDs: 1x Red, 1x White
 - Serial console: internal test points, 115200 8n1
 - Power: 5 VDC, 3 A

MAC addresses:
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| WAN     | 80:af:ca:xx:xx:x1 | label+1   |
| LAN     | 80:af:ca:xx:xx:x0 | label     |
| WLAN 2g | 80:af:ca:xx:xx:x0 | label     |
| WLAN 5g | 82:af:ca:xx:xx:x0 |           |
+---------+-------------------+-----------+

Installation:
The installation must be done via TFTP by disassembling the router. On other occasions Cudy has distributed intermediate firmware to make installation easier, and so I recommend checking the Wiki for this device if there is a more convenient solution than the one below.

To install using TFTP:
1. Connect to UART.
2. 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.
3. A u-boot shell will automatically open.
4. Connect to LAN and set your IP to 192.168.1.88/24. Configure a TFTP server and an OpenWrt initramfs-kernel.bin firmware file.
5. Run these steps in u-boot using the name of your file.

setenv bootfile initramfs-kernel.bin
tftpboot
bootm

6. If you can reach LuCI or SSH now, just use the sysupgrade image with the 'Keep settings' option turned off.

Signed-off-by: Luis Mita <luis@luismita.com>