realtek: mdio: RTL838x: move functions over to bus

realtek: mdio: RTL838x: move functions over to bus

The mdio bus functions are still split between ethernet and dsa driver.
Before moving everthing out to a separate mdio driver we decided to
collect everything in the ethernet driver with the rtmdio prefix.
Take over the remaining RTL838x functions.

Remark: This is more or less a copy/paste with function renaming. As
there are still some consumers in the DSA driver the definitions and
inclusions must be flipped.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19484
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>