mediatek: filogic: add support for Cudy M3000 v1

mediatek: filogic: add support for Cudy M3000 v1

Hardware:
  SoC:     MT7981b
  RAM:     256 MB
  Flash:   128 MB SPI NAND
  Ethernet:
    1x 2.5Gbps (rtl8221b)
    1x 1Gbps (integrated phy)
  WiFi:    2x2 MT7981
  Buttons: Reset, WPS
  LED:     1x multicolor

Solder on UART:
  - remove rubber ring on the bottom
  - remove screws
  - pull up the cylinder, maybe help by push on an ethernet socket with a screwdriver
  - remove the (3) screws holding the board in the frame
  - remove the board from the frame to get to the screws for the silver, flat heat shield
  - remove the (3) screws holding the heat shield
  - solder UART pins to the back of the board
    - make sure to have the pins point out on side with the black, finned heat spread
    - the markings for the pins are going to be below the silver heat shield
    - Vcc is not needed

If you don't intend on using the UART outside of the installation process, you might not
want to solder:

  - carefully scrape off the thin layer of epoxy on the holes (not the copper)
  - place your pin header with the UART attached in the holes
  - the pins, starting with the one closest to the socket:
    - Vcc (not required)
    - GND
    - RX
    - TX
  - either wedge the header or hold it with your fingers so that the pins stay in contact with the board

Installation (UART):
  - attach an Ethernet cable to the 1Gbps port (black) on the router
  - hold the reset button while powering the router
  - press CTRL-C or wait for the timeout to get to the U-Boot prompt
  - prepare a TFTP server on the network to supply ..-initramfs-kernel.bin
  - use 'tftpboot' in the U-Boot shell to pull the image
  - boot the image using 'bootm'
  - push the ..-sysupgrade to the router using your preferred method
  - perform the upgrade with 'sysupgrade -n'

There is a recovery mechanism that involves fetching a file called 'recovery.bin' but that is not understood yet.

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>