Adding support for Zyxel GS1900-48HPv2

[Deeily]

Hello OpenWrt developers,

I am writing to contribute a preliminary device tree source (DTS) file
for the Zyxel GS1900-48HP switch. I am a beginner in this area, but I
managed to create a working DTS by using the DTS for the GS1900-24HPv2
model as a base.

With this DTS, the device boots and basic switching functionality
works. Power over Ethernet (PoE) is supplied on all 24 PoE-capable
ports, however, I have identified a significant issue with PoE
management:

    PoE Control Issue: Currently, it is only possible to control the
power (on/off) for the first PoE port (port 0). The remaining 23 PoE
ports are always on and cannot be managed via software.

I understand that this is an incomplete patch and the PoE control
needs to be fixed. However, I am providing it as a starting point. I
am willing to help in any way I can.

I am ready to test new firmware builds and provide any additional
data, outputs from dmesg, logread, or information from ubus that might
be helpful for debugging the PoE control issue.

The DTS file is attached to this email. Thank you for your time and
for the great work you do on OpenWrt.

Best regards, Kirill
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