realtek: add PoE enable line to Netgear GS310TP

By switching to the new RTL8231 driver in commit b7af54d5c1 ("realtek:
Simple conversions to RTL8231 MFD driver"), the bootloader state of the
RTL8231's pins is now maintained. As the bootloader de-asserts the PoE
enable signal, this means PoE output is no longer available.

Add a gpio-hog with high output, restoring the line value from when the
pin was configured (by default) as an input with a pull-up resistor.
This will hard-enable the PoE output, but the individual ports can still
be administratively disabled by realtek-poe or a similar tool.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
(cherry picked from commit 890293c13c)

target/linux/realtek/dts/rtl8380_netgear_gs310tp-v1.dts

@@ -8,6 +8,15 @@
 
 };
 
+&gpio1 {
+	poe-enable {
+		gpio-hog;
+		gpios = <10 GPIO_ACTIVE_HIGH>;
+		output-high;
+		line-name = "poe-enable";
+	};
+};
+
 &firmware {
 	openwrt,ih-magic = <0x4e474335>;
 };