filogic: add support for ASUS TUF-AX4200

filogic: add support for ASUS TUF-AX4200

Hardware
--------
SOC:   MediaTek MT7986
RAM:   512MB DDR3
FLASH: 256MB SPI-NAND (Winbond W25N02KV)
WIFI:  Mediatek MT7986 DBDC 802.11ax 2.4/5 GHz
ETH:   MediaTek MT7531 Switch
       MaxLinear GPY211C 2.5 N-Base-T PHY
UART:  3V3 115200 8N1 (Pinout silkscreened / Do not ocnnect VCC)

Installation
------------

1. Download the OpenWrt initramfs image. Copy the image to a TFTP server
   reachable at 192.168.1.66/24. Rename the image to tufax4200.bin.

2. Connect the TFTP server to the AX4200. Conect to the serial console,
   interrupt the autoboot process by pressing '4' when prompted.

3. Download & Boot the OpenWrt initramfs image.

   $ setenv ipaddr 192.168.1.1
   $ setenv serverip 192.168.1.66
   $ tftpboot 0x46000000 tufax4200.bin
   $ bootm 0x46000000

4. Wait for OpenWrt to boot. Transfer the sysupgrade image to the device
   using scp and install using sysupgrade.

   $ sysupgrade -n <path-to-sysupgrade.bin>

Missing features
----------------

 - The LAN port LEDs are driven by the switch but OpenWrt does not
   correctly configure the output.

Signed-off-by: David Bauer <mail@david-bauer.net>