realtek: rtl838x: drop GS1900 MDIO reset GPIO

realtek: rtl838x: drop GS1900 MDIO reset GPIO

The reset line wired to the RTL8231 on the GS1900 series may also
connect to other external ICs on the board. On the GS1900-24E, the
reset line is wired (via buffers) to the board's RTL8231 expanders and
the RTL8218 phys. As these external devices (phys) are on different
busses, the reset line shouldn't be specified on one bus or the other.

Drop the reset specification from the generic GPIO description, so it
can be added back on a per-device basis after confirming the behavior.

Link: https://github.com/openwrt/openwrt/issues/18620
Fixes: fd978c2e80b4 ("realtek: Enable Zyxel GS1900's RTL8231 reset line")
Signed-off-by: Sander Vanheule <sander@svanheule.net>