net: mvpp2: mdio device per port

Current code forces all ports on a given Ethernet device to use the same
mdio device. In practice different ports might be wired to separate mdio
devices. Move the mdio device from the container struct mvpp2 to the per
port struct mvpp2_port.

Cc: Ken Ma <make@marvell.com>
Cc: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>