net: mscc: refactor mscc_miim

Because all MSCC SoC use the same MDIO bus, put the implementation in
one common file(mscc_miim) and make all the other MSCC network drivers to
use these functions.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>