mediatek: add support for WAVLINK WL-WN536AX6 Rev a

mediatek: add support for WAVLINK WL-WN536AX6 Rev a

Product name: Wavlink WL-WN536AX6 Rev a "Mighty LX2"
link: https://www.wavlink.com/en_us/product/WL-WN536AX6.html
Specifications:
SOC: MT7986AV
RAM: 512MB DDR4
Flash: 128MB SPI NAND
Ports: 4 LAN (1G) & 1 WAN (2.5G)
WIFI: MT7976PN + MT7975N
LEDs: 8 (STATUS, WIFI, WAN, LAN1, LAN2, LAN3, LAN4, PWR)
USB: 1 (3.0)

MAC table, same as stock firmware:
LAN: 80:3F:5D:xx:xx:x1 partition "hw" at 0x44e (ASCII)
WAN: 80:3F:5D:xx:xx:x2 partition "hw" at 0x460 (ASCII)
2G: 80:3F:5D:xx:xx:x3
5G: 80:3F:5D:xx:xx:x4

* Installation with OEM WebUI:

Note: Make sure PC is connected on LAN1 port. The OEM firmware has an unknown root password and settings are kept after upgrading firmware. Therefore, a customized Openwrt firmware is needed to remove the root password on login, by adding `passwd -d root` to /etc/init.d/bootcount. The WebUI does a filename check so the customized firmware is named accordingly.

1. Download modified firmware file `WAVLINK_WN536AX6-A_M36AX6_V250320-WO-437baca-modified.bin` from https://github.com/ses1er/firmware-misc/tree/main/wavlink/wl-wn536ax6a
2. Log into WebUI on default IP: http://192.168.20.1
3. Browse to More (top menu) -> System -> Firmware Upgrade.
4. Under `Local Upgrade` section, check the device to be upgraded and upload downloaded modified firmware. Click `UPLOAD FILE`, then `APPLY`
5. Wait about 2 minutes (ignore progress bar), and browse to http://192.168.20.1. You should see LUCI login page. Username is root and no password.
6. Browse to `System -> Backup/Flash Firmware`, click on `Flash Image`, click `Browse` and locate `openwrt-mediatek-filogic-wavlink_wl-wn536ax6-a-squashfs-sysupgrade.bin` file.
7. Uncheck `Keep settings and retain the current configuration` and click `Continue`.
8. Router will now be set to IP 192.168.1.1 which is the Openwrt default.

* Installation with UART:

Note: Having UART connected while cold booting the device will result in a kernel panic when initializing wifi. I've found this workaround:
1. Power off the device and ensure UART is not connected to PC.
2. Power up the device, when lights come on, plug in UART.
3. Warm boots and soft restarts will not cause kernel panic for the duration of device being powered on. Repeat steps for subsequent cold boots.

1. Configure TFTP server with IP 192.168.1.66. Copy `openwrt-mediatek-filogic-wavlink_wl-wn536ax6a-initramfs.itb` to TFTP root.
2. Interrupt boot by pressing 0.
3. Run the following in Uboot console: `setenv serverip 192.168.1.66; setenv ipaddr 192.168.1.1; tftpboot 0x46000000 openwrt-mediatek-filogic-wavlink_wl-wn536ax6-a-initramfs.itb; bootm`
4. Transfer `openwrt-mediatek-filogic-wavlink_wl-wn536ax6-a-squashfs-sysupgrade.bin` to device:
   (`scp -O openwrt-mediatek-filogic-wavlink_wl-wn536ax6-a-squashfs-sysupgrade.bin [email protected]:/tmp/`)
5. Run the following on device: `sysupgrade -n /tmp/openwrt-mediatek-filogic-wavlink_wl-wn536ax6-a-squashfs-sysupgrade.bin`

Signed-off-by: Qing W. <[email protected]>
Tested-by: Martin Blumenstingl <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20760
Signed-off-by: Hauke Mehrtens <[email protected]>