realtek: mdio: rtl931x: move functions over to bus

realtek: mdio: rtl931x: move functions over to bus

This commit repeats the mdio function relocation from the other targets.
In short that means:

- phy read/write functions are moved away from the phy driver
- SerDes read/write functions are moved away from the dsa driver
- All gets consolidated into the mdio driver (inside the ethernet driver)

This is mostly a copy/paste to keep the changes small. The SerDes phy mapping
and the simplification of the central bus functions will come later.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19743
Signed-off-by: Robert Marko <robimarko@gmail.com>