net: phy: Broadcom iProc MDIO bus driver

This patch adds support for the Broadcom iProc MDIO bus interface.
The MDIO interface can be found in the Broadcom iProc family Soc's.

The MDIO bus is accessed using a combination of command and data
registers. This MDIO driver provides access to the Etherent GPHY's
connected to the MDIO bus.

Signed-off-by: Arun Parameswaran <arunp@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>