realtek: carve out mdio bus from ethernet driver

realtek: carve out mdio bus from ethernet driver

So much code was distributed between phy/ethernet/dsa drivers. A lot
was already cleand up before. With this step the mdio bus gets its
own space and is no longer hidden inside the ethernet driver.

This commit is mostly a copy/paste that includes only minor changes.

- define prefixes are renamed to RTMDIO
- The driver is totally self contained (does not rely on SoC include)
- The DTS structure (mdio node below ethernet node) was kept
- The driver is added to the kernel config of all subtargets.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19942
Signed-off-by: Robert Marko <robimarko@gmail.com>