realtek: add support for ZyXEL GS1920-24HPv1

realtek: add support for ZyXEL GS1920-24HPv1

The GS1920-24HPv1 is a switch with 24 copper ports and 4 combo SFP/copper
ports and PoE on the first 24 ports.

Specifications:
---------------
  * SoC: Realtek RTL8292M
  * Flash: 16 MiB SPI flash
  * RAM: 128 MiB
  * Ethernet: 24x 10/100/1000 Mbps
  * Buttons: 1x "Reset" button
  * UART: 1x serial header, standard DCE pinout (Tx = 2, Rx = 3, Gnd = 5);
          9600 baud, 8n1, +- 5.6V logic levels
  * SFP: 4 combo copper/SFP ports
  * PoE: 24x
  * Fans: ADT7468 fan controller

Works:
------
  - (24) RJ-45 ethernet ports
  - Switch functions
  - Buttons
  - LEDs (partial support, the wrong LEDs light up)
  - Manual fan control

Not yet enabled:
----------------
  - PoE (requires patches to realtek-poe to support i2c)
  - Combo ports (link is up, but no data is transferred)

Fans:
-----
After boot, the fans are running in full speed mode. You can interact
with the fan controller at /sys/class/hwmon/

Installation:
-------------

This device uses ZyNOS instead of Linux, this makes installation a bit
more cumbersome. Serial console is required!

1. Set the switch to boot from the first image. This step is crucial,
   it will fail to boot if this is not set properly.

2. Connect to the switch using serial and interrupt the boot process
   to enter debug/recovery mode.

3. Load the OpenWrt initramfs image via XMODEM. You need to obtain an
   unlock code, based on your MAC address, first. See the excellent write
   up at https://www.ixo.de/info/zyxel_uclinux/ for details. Replace
   unlock_code in the commands below by the code obtained.
   After running ATBA5, the terminal needs to be closed and re-opened
   with 115200 baud. This speeds up the file transfer significantly!
   The file length in bytes need to be given instead of file_length below.
   You also need an XMODEM upload utility like "lrzsz-sx -X" to transfer
   the file. Start the XMODEM upload after running the ATUPxxxx command:

     > ATEN1,unlock_code
     > ATBA5
     > ATUP80100000,file_length
     > ATGO80100000

4. Wait for OpenWrt to boot. Once this is done, transfer the loader binary
   and the sysupgrade image to "/tmp" using scp.

5. Install OpenWrt permanently by running the following two commands on
   the switch (over SSH):

    > mtd write /tmp/loader.bin loader
    > mtd write /tmp/sysupgrade.bin firmware

6. Reboot the switch and enjoy OpenWrt.

NB: You do not need to touch the loader binary unless it's recommended.
    The loader is not part of a regular sysupgrade file and will be left
    untouched. The boot loader only checks if the loader is valid to be
    able to boot.

Recovery/ Return to stock:
--------------------------

Just spam the "u" key during (or "z" for 9600 baud) during memory testing
to trigger a recovery XMODEM upload at 115200 baud. A standard OEM upgrade
image works properly.

Signed-off-by: Andreas Böhler <dev@aboehler.at>
Link: https://github.com/openwrt/openwrt/pull/20439
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>