realtek: add ZyXEL GS1900-24HPv2 support

The ZyXEL GS1900-24HPv2 is a 24 port PoE switch with two SFP ports, similar to the other GS1900 switches.

Specifications
--------------
* Device:    ZyXEL GS1900-24HPv2
* SoC:       Realtek RTL8382M 500 MHz MIPS 4KEc
* Flash:     16 MiB
* RAM:       W631GG8MB-12 128 MiB DDR3 SDRAM
             (stock firmware is configured to use only 64 MiB)
* Ethernet:  24x 10/100/1000 Mbps, 2x SFP 100/1000 Mbps
* LEDs:      1 PWR LED (green, not configurable)
             1 SYS LED (green, configurable)
             24 ethernet port link/activity LEDs (green, SoC controlled)
             24 ethernet port PoE status LEDs
             2 SFP status/activity LEDs (green, SoC controlled)
* Buttons:   1 "RESTORE" button on front panel
             1 "RESET" button on front panel
* Power      120-240V AC C13
* UART:      1 serial header (J41) with populated standard pin connector on
             the left edge of the PCB, angled towards the side.
             The casing has a rectangular cutout on the side that provides
             external access to these pins.
             Pinout (front to back):
             + GND
             + TX
             + RX
             + VCC

Serial connection parameters for both devices: 115200 8N1.

Installation
------------

OEM upgrade method:

(Possible on master once https://patchwork.ozlabs.org/project/openwrt/patch/20210624210408.19248-1-bjorn@mork.no/ is merged)

* Log in to OEM management web interface
* 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 openwrt-realtek-generic-zyxel_gs1900-24hp-v2-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 image to /tmp and flash it:
   > sysupgrade -n /tmp/openwrt-realtek-generic-zyxel_gs1900-24hp-v2-squashfs-sysupgrade.bin
   it may be necessary to restart the network (/etc/init.d/network restart) on
   the running initramfs image.

U-Boot TFTP method:

* Configure your client with a static 192.168.1.x IP (e.g. 192.168.1.10).
* 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 GS1900-24HPv2 is a dual-partition device, you want to keep the OEM
  firmware on the backup partition for the time being. OpenWrt can only boot
  from 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 192.168.1.10:openwrt-realtek-generic-zyxel_gs1900-24hp-v2-initramfs-kernel.bin
   > bootm
* Once OpenWrt has booted, scp the sysupgrade image to /tmp and flash it:
   > sysupgrade -n /tmp/openwrt-realtek-generic-zyxel_gs1900-24hp-v2-squashfs-sysupgrade.bin
   it may be necessary to restart the network (/etc/init.d/network restart) on
   the running initramfs image.

Signed-off-by: Soma Zambelly <zambelly.soma@gmail.com>

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