realtek: phy: add RTL8218D initialization

realtek: phy: add RTL8218D initialization

The RTL8218D currently relies on proper U-Boot configuration. In
case that is not possible, provide a basic setup sequence that can
bring the PHY "alive". The SDK provides multiple configuration
sequences for two operation modes (XSGMII or QSGMII) and the different
SoC families. Due to limited testing resources only provide a setup
for RTL93xx devices and both modes at the moment.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21551
Signed-off-by: Robert Marko <robimarko@gmail.com>