realtek: add support for Zyxel GS1900-10HP B1

realtek: add support for Zyxel GS1900-10HP B1

The GS1900-10HP B1 keeps the RTL8380M SoC and port layout but replaces the
A1's Broadcom PoE (Gen1) with a Realtek RTL8238 PSE fronted by a Nuvoton
MCU speaking the Gen2 protocol at 115200 baud. It reuses the shared
rtl8380_zyxel_gs1900-10hp.dtsi and only overrides the PSE compatible and
UART speed.

Specifications:
- SoC: Realtek RTL8380M
- Ports: 8x 10/100/1000 (PoE+), 2x SFP
- PoE: Realtek RTL8238 via Nuvoton MCU (Gen2, UART 115200)

Tested on hardware: driver probes RTL8238B (8 ports), per-port control via
ethtool, verified across a cold power-cycle.

Signed-off-by: Carlo Szelinsky <github@szelinsky.de>
Link: https://github.com/openwrt/openwrt/pull/24283
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>