realtek: phy: add RTL8214FC initialization for RTL839x

realtek: phy: add RTL8214FC initialization for RTL839x

Until now the RTL8214FC is initialized either by U-Boot (all
devices) or by some magic firmware file (RTL838x). On RTL839x
devices without U-Boot (e.g. ZyXEL GS1920) this PHY cannot
be used.

Provide the most basic setup sequence for RTL839x so that
copper/fiber work. Later it can be taken over for all devices
and the firmware helpers can be dropped.

Remark! This should not (but might) break RTL839x devices with
RTL8214FC U-Boot setup.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21435
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>