realtek: mdio: create new bus structure

realtek: mdio: create new bus structure

The port specific attributes of the central control structure
have been moved over to a separate port structure. Do the same
for the (up to 4) busses. Establish a separate bus structure
that will hold data about each bus and use it in the control
structure. As a first usage type move the is_c45 attribute over.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22604
Signed-off-by: Robert Marko <robimarko@gmail.com>