realtek: rtl931x: psx28: specify POE MCU reset GPIO

The MCU (GD32E230G8) which controls the RTL8239 POE++ PSE chips can
sometimes hang. In this case, it is necessary to to reset the chip using
the nRESET pin which is connected to the GPIO1 of the RTL8231 GPIO
expander.

For a reset, the `/sys/class/gpio/poe_mcu_reset/value` file must be set to
1 for a short period and then back to 0. After that, the poemgr must be
"restarted" to the MCU back in the expected state.

Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/22916
Signed-off-by: Robert Marko <[email protected]>

target/linux/realtek/dts/rtl9312_plasmacloud_psx28.dts

@@ -14,6 +14,16 @@
 			gpios = <&gpio1 28 GPIO_ACTIVE_LOW>;
 		};
 	};
+
+	gpio-export {
+		compatible = "gpio-export";
+
+		poe_mcu_reset {
+			gpio-export,name = "poe_mcu_reset";
+			gpio-export,output = <1>;
+			gpios = <&gpio1 29 GPIO_ACTIVE_LOW>;
+		};
+	};
 };
 
 &i2c_mst1 {