Enable FEC driver to retrieve PHY address from device tree

Currently if we have more than one phy on the MDIO bus, we do not have a
good mechanism for determining which should be used at runtime. Enable the
FEC driver to determine the address for the PHY from the device tree.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>