realtek: phy: add 100base-FX mode to RTL8214FC

realtek: phy: add 100base-FX mode to RTL8214FC

Add the basic bits to allow 100base-FX SFP mode on the RTL8214FC.
While this looks good fom ethtool perspective, it does not really
change the phy registers to enforce the mode. The SFP is still
driven in 1000base-X.

While it might seem useless at the moment this at least opens
up a new phy control method. This comes handy with one known bug.
In rare cases a SFP that is plugged in during boot does not get
a link. One option to revive the dead port seems to be

root@OpenWrt:~#  ethtool -s lan28 speed 100 duplex full autoneg off
rtl83xx-switch 1b000000.switchcore:ethernet-switch lan28: Link is Up - 100Mbps/Full - flow control off
switch: port 28(lan28) entered blocking state
switch: port 28(lan28) entered forwarding state
rtl83xx_fib_event: FIB_RULE ADD/DEL for IPv6 not supported
rtl83xx_fib_event: FIB_RULE ADD/DEL for IPv6 not supported

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/23087
Signed-off-by: Robert Marko <robimarko@gmail.com>