mediatek: add support for YunCore AX835

mediatek: add support for YunCore AX835

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: 16MB NOR
  RAM: 256MB
  Ethernet: 2x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset
  Power: DC 12V 1A, PoE 802.3af 48V

Flash instructions:

Option #1 - SSH

  I was able to SSH into the stock firmware of my device.

  1. Attach the router to the network
  2. Use scp (-O) to copy the sysupgrade image
  3. Connect using SSH and run `sysupgrade -n`

Option #2 - U-Boot

  One way to use the bootloader for flashing is using TFTP:

  1. Connect to the router using an ethernet cable
  2  Spin up a TFTP server serving the sysupgrade file
  3. Open the case and attach a UART
  4. Attach power to the router and interrupt the countdown by pressing
     any key
  5. Select option #2 (Upgrade firmware)
  6. Enter IP address information and image name
  7. Wait patiently

Co-Authored-By: Enrique Rodríguez Valencia <enrique.rodriguez@galgus.net>
Co-Authored-By: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>
(cherry picked from commit b4086f44cd8a739458a0fd12cfaf684515507614)