fm-eth: add function fm_info_get_phy_address()

Function fm_info_get_phy_address() returns the PHY address for a given
Fman port.  This is handy when the MDIO code needs to fixup the Ethernet
nodes in the device tree to point to PHY nodes for a specific PHY address.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>