mediatek: add support for OpenFi 6C

mediatek: add support for OpenFi 6C

OpenFi 6C is a portable Wi-Fi 6 travel router based on MediaTek MT7981B+MT7976CN.

Two slightly different versions have been sold. The V1 board has a green color and lacks the modem LED. The V2 board is black and has a LED for the modem. The firmware should work on both of them.

Specifications:
- SoC: MediaTek MT7981B (Filogic 820) 1.3GHz dual-core ARM Cortex-A53
- RAM: 1GB DDR4
- Flash: 256MB SPI NAND
- Wireless: 2.4GHz/5GHz 802.11ax
- Ethernet: 1x 10/100/1000M LAN
- USB: 1x USB 3.0 Type-A port
- Expansion: M.2 slot for 5G modem
- Cooling: PWM-controlled fan
- Buttons: Reset, Mode switch
- LEDs: System, Ethernet, 5G WiFi, Modem status

**Installation via U-Boot web page**

1. Set static IP 192.168.21.2/255.255.255.0 on your computer.
2. Connect to the Ethernet port and hold the reset button while booting the device. Wait for 6-8 seconds, and release the reset button.
3. Open U-boot web page on your browser at http://192.168.21.1
4. Select the OpenWRT sysupgrade image, upload it, and start the upgrade.
5. Wait for automatic reboot.

**Installation via sysupgrade**

Flash the sysupgrade file via LuCI upgrade page without saving the settings.

Signed-off-by: Jiasheng Zhu <newbanyaya@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19512
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>