mediatek: add support for Mercusys MR80X v3

mediatek: add support for Mercusys MR80X v3

This commit adds support for Mercusys MR80X(EU) v3 router.

Device specification:
 - SoC: Mediatek MT7981b, Cortex-A53, 64-bit
 - RAM: 512MB
 - Flash: SPI NAND GigaDevice GD5F1GQ5UEYIGY (128 MB)
 - Ethernet: 4x 100/1000 Mbps LAN1,LAN2,LAN3 & WAN
 - Wireless: 2.4GHz (802.11 b/g/n/ax)
 - Wireless: 5GHz (802.11 a/n/ac/ax)
 - LEDs: 1 orange and 1 green status LEDs, 4 green gpio-controlled LEDs
   on ethernet ports
 - Buttons: 1 (Reset)
 - Bootloader: Main U-Boot - U-Boot 2022.01-rc4. Additionally, both UBI
   slots contain "seconduboot" (also U-Boot 2022.01-rc4)

Installation (UART):
 - Place OpenWrt initramfs-kernel image on tftp server with IP 192.168.1.2
 - Attach UART, switch on the router and interrupt the boot process by
   pressing 'Ctrl-C'.
 - Set the uboot environment for startup.
   setenv tp_boot_idx 0; setenv bootcmd bootm 0x46000000; saveenv
   If the bootarg is set to boot from ubi1, also change it to ubi0.
 - Load and run OpenWrt initramfs image.
   setenv serverip 192.168.1.2; setenv ipaddr 192.168.1.1; tftpboot initramfs-kernel.bin; bootm
 - Browse IP 192.168.1.1, upload the 'sysupgrade' image and do upgrade.

Recovery:
 - Press Reset button and power on the router.
 - Navigate to U-Boot recovery web server (http://192.168.1.1/) and
   upload the OEM firmware.

Stock layout:
0x000000000000-0x000000200000 : "boot"
0x000000200000-0x000000300000 : "u-boot-env"
0x000000300000-0x000003500000 : "ubi0"
0x000003500000-0x000006700000 : "ubi1"
0x000006700000-0x000006f00000 : "userconfig"
0x000006f00000-0x000007300000 : "tp_data"

ubi0/ubi1 format:
U-Boot at boot checks that all volumes are in place:
+-------------------------------+
| Volume Name: uboot   Vol ID: 0|
| Volume Name: kernel  Vol ID: 1|
| Volume Name: rootfs  Vol ID: 2|
+-------------------------------+

MAC addresses:
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| label   | 94:0C:xx:xx:xx:12 | label     |
| WAN     | 94:0C:xx:xx:xx:13 | label+1   |
| LAN     | 94:0C:xx:xx:xx:12 | label     |
| WLAN 2g | 94:0C:xx:xx:xx:11 | label-1   |
| WLAN 5g | 94:0C:xx:xx:xx:10 | label-2   |
+---------+-------------------+-----------+
label MAC address was found in UBI partition "tp_data", file
"default-mac".

Signed-off-by: Schneider Azima <Schneider-Azima12@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/18181
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>