net: mdio: Add helper functions for accessing MDIO devices

This adds some helpers for accessing non-phy MDIO devices. They are
analogous to phy_(read|write|modify), except that they take an mdio_device
and not a phy_device.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>