realtek: add support for ZyXEL GS1900-8HP B2

realtek: add support for ZyXEL GS1900-8HP B2

The ZyXEL GS1900-8HP B2 is an 8-port gigabit switch with PoE+ support. It's a new
hardware revision that uses the `realtek,pse-mcu-gen2` PSE dialect with 115200 baud
like the GS1900-10HP B1. Other hardware changes are unknown, but the switch works
fine using the pre-PSE `zyxel,gs1900-8hp-b1` image with the `realtek-poe` user-space
PoE implementation.

The installation instructions from the initial support for the A1 and B1 revisions
still apply (https://github.com/openwrt/openwrt/commit/c4bfe68c83910e613f07350587a8709a16bd1ffa):

* Configure your client with a static 192.168.1.x IP (e.g. 192.168.1.2).
* Set up a TFTP server on your client and make it serve the initramfs
  image.
* Connect serial, power up the switch, interrupt U-boot by hitting the
  space bar, and enable the network:
  > rtk network on
* Since the GS1900-10HP is a dual-partition device, you want to keep the
  OEM firmware on the backup partition for the time being. OpenWrt can
  only boot off the first partition anyway (hardcoded in the DTS). To
  make sure we are manipulating the first partition, issue the following
  commands:
  > setsys bootpartition 0
  > savesys
* Download the image onto the device and boot from it:
  > tftpboot 0x84f00000 192.168.1.2:openwrt-realtek-generic-zyxel_gs1900-8hp-b2-initramfs-kernel.bin
  > bootm
* Once OpenWrt has booted, scp the sysupgrade image to /tmp and flash it:
  > sysupgrade /tmp//tmp/openwrt-realtek-generic-zyxel_gs1900-8hp-b2-squashfs-sysupgrade.bin

Alternatively, after factory-resetting the switch, you can:

* Log in to OEM management web interface. It should be at http://192.168.1.1
* Navigate to `Maintenance > Firmware > Management`
* If "Active Image" has the first option selected, OpenWrt will need to be
  flashed to the "Active" partition. If the second option is selected, OpenWrt
  will need to be flashed to the "Backup" partition.
* Navigate to `Maintenance > Firmware > Upload`
* Upload the initramfs-kernel.bin file by your preferred method to the previously
  determined partition. When prompted, select to boot from the newly flashed image,
  and reboot the switch.
* Once OpenWrt has booted, scp the sysupgrade.bin image to /tmp and flash it thought SSH.
  - OpenWrt does not include openssh-sftp-server by default. If your SCP client fails due
    to lacking an SFTP server on the device, consider using the legacy SCP protocol
    instead. With OpenSSH's scp this can be done by adding the -O option on the command
    line.
  - `sysupgrade -n /tmp/<sysupgrade file name>`
* It may be necessary to restart the network (/etc/init.d/network restart) on the running
  initramfs image.

See the GS199-8HP A1 wiki page for more information (https://openwrt.org/toh/zyxel/gs1900-8hp_v1).

Signed-off-by: Peter Putzer <openwrt@mundschenk.at>
Link: https://github.com/openwrt/openwrt/pull/24418
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>