phylib: add mdiobus_{read,write}

Add mdiobus_{read,write} routines to allow direct reading/writing
of registers on an mii bus without having to go through the PHY
abstraction, and make phy_{read,write} use these primitives.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>