mediatek: filogic: add support for ASUS RT-AX52

mediatek: filogic: add support for ASUS RT-AX52
Hardware
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SOC:   MediaTek MT7981b
RAM:   256MB DDR3
FLASH: 128MB SPI-NAND (Winbond W25N01GV)
WIFI:  Mediatek MT7981b DBDC 802.11ax 2.4/5 GHz
ETH:   MediaTek MT7531 Switch
UART:  3V3 115200 8N1 (Pinout silkscreened / Do not connect VCC)

Installation
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Vendor-UI Method
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1. Download the OpenWrt initramfs.trx image.

2. Connect the PC via LAN to one of the yellow router ports and wait
   until your PC to get a DHCP lease.

3. Browse to http://192.168.50.1

4. If your router is brand new, finish the setup process and log into
   the Web-UI.

5. Navigate to Administration -> Firmware Upgrade and upload the
   downloaded OpenWrt image.

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

   $ sysupgrade -n <path-to-sysupgrade.bin>
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TFTP Method
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1. Download the OpenWrt initramfs image. Copy the image to a TFTP server
   reachable at 192.168.1.70/24. Rename the image to rtax52.bin.

2. Connect the PC with TFTP server to the RT-AX52.
   Set a static ip on the ethernet interface of your PC.
     (ip address: 192.168.1.70, subnet mask:255.255.255.0)
  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 192.168.1.70
   $ tftpboot 0x46000000 rtax52.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>
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Revert to stock firmware:
1: Download the rt-ax52 firmware from ASUS official website. Save the firmware to tftp server directory and rename
   to RT-AX52.trx

2: Connect the PC with TFTP server to the RT-AX52.
   Set a static ip on the ethernet interface of your PC.
     (ip address: 192.168.1.70, subnet mask:255.255.255.0)

3: Conect to the serial console,  power on again,  interrupt the autoboot process by pressing '4' when prompted.
  $: ubi remove linux
  $: ubi remove jffs2
  $: ubi remove rootfs
  $: ubi remove rootfs_data
  $: ubi create linux 0x45fe000
  $: reset
  then the dut will reboot,interrupt the autoboot process by pressing '2' when prompted.
            2: Load System code then write to Flash via TFTP.
             Warning!! Erase Linux in Flash then burn new one. Are you sure?(Y/N)
             $: enter y
  you will see the follow, type enter directly:
        Input device IP (192.168.1.1) ==:
        Input server IP (192.168.1.70) ==:
        Input Linux Kernel filename (RT-AX52.trx) ==:

4: wait for the device run up

(based on support for ASUS RT-AX52 by liudongdongdong7397 and trx image
generation by remittor)

Signed-off-by: Christoph Krapp <achterin@gmail.com>