phy: Add Northstar2 PCI Phy support

Add PCI Phy support for Broadcom Northstar2 SoCs.  This driver uses the
interface from the iproc mdio mux driver to enable the devices
respective phys.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>