realtek: add support for Zyxel XGS1930-28HP

realtek: add support for Zyxel XGS1930-28HP

Add support for the RTL9301-based Zyxel XGS1930-28HP, a 28-port Gigabit
PoE+ switch. The XGS1930 is an EOL Zyxel series of RTL9301-based
switches available with 28 or 52 ports, with and without PoE.

Hardware
========

  - RTL9301 SoC
  - 512 MiB DDR3 RAM
  - 32 MiB SPI-NOR flash
  - 24x 10/100/1000M RJ45 ports
  - 4x 1G/10G SFP+ cages
  - PoE:
    - 802.3af/at on all 24 RJ45 ports
    - 375 W total power budget
  - RTL8231 for port LEDs
  - Front LEDs: PWR, SYS, CLOUD, LOCATOR, PoE usage bar (5 steps)
  - Buttons: 1x "Restore"
  - Console: TTL 3.3V, 115200 8N1, 4-pin header
    - pinout (front to back): GND RX TX -
  - Software chain:
    - Bootbase/stripped-down U-Boot
    - RAS/ZyNOS

MAC address
===========

Single MAC address derived from the board partition. Applied to all
switch ports.

Disclaimer
==========

PoE is not yet supported.

Flashing OpenWrt overwrites ZyNOS. The Bootbase/U-Boot remains intact
and can be used for recovery.

Installation
============

Simple web upgrade:

1. Take the OpenWrt factory.bin image generated by the build.

2. In the ZyNOS web UI, login and go to Maintenance -> Firmware Upgrade.

3. If the device runs ZyNOS 5.00, untick "Enhanced firmware integrity
   check sha256sum". Otherwise the upload check will reject the image.

4. Select and upload the factory.bin image and click upgrade.

5. After flashing has finished, reboot the switch. It will now boot
   into OpenWrt.

Initramfs boot
==============

Luckily the switch uses a standard design, thus networking works with
a default hardware profile of RTK U-boot.

1. Connect to the serial console and interrupt the boot process by
   spamming '$' during the DRAM test to drop into Bootbase/U-Boot.

2. Bring up the network:

   > rtk network on

   Use a copper port; the SFP+ cages are likely not usable from the
   bootloader.

3. Load the initramfs image via TFTP:

   > tftpboot 0x82000000 <server>:<image>

4. Run the image (not bootm, the image has no uImage header):

   > go 0x82000000

Return to stock firmware
========================

1. Download the stock firmware for the switch from the Zyxel website
   and unzip it; there should be a .bin file with an alphanumeric name.

2. Upload that file to the running OpenWrt instance.

3. Flash it (use -F since the image has no OpenWrt metadata):

   > sysupgrade -F <stock-firmware>.bin

4. Wait for sysupgrade to finish and the switch to reboot. ZyNOS should
  come up again.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23389
Signed-off-by: Robert Marko <robimarko@gmail.com>