enetc: Add ENETC PF level external MDIO support

enetc: Add ENETC PF level external MDIO support

Each ENETC PF has its own MDIO interface, the corresponding
MDIO registers are mapped in the ENETC's Port register block.
The current patch adds a driver for these PF level MDIO buses,
so that each PF can manage directly its own external link.

Signed-off-by: Alex Marginean <[email protected]>
Signed-off-by: Claudiu Manoil <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Signed-off-by: David S. Miller <[email protected]>