realtek: add PoE enable line to Netgear GS310TP

realtek: add PoE enable line to Netgear GS310TP

By switching to the new RTL8231 driver in commit b7af54d5c18c ("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>