stmmac: platform: fix FIXED_PHY support.

On several STi platforms: e.g. stihxxx-b2120 an Ethernet switch is
embedded and connected to the stmmac via RGMII mode. So this is managed
by using the FIXED_PHY. In that case, the support in the platform needs
to be fixed to allow the stmmac to dialog with the switch via fixed-link
by using phy_bus_name property.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>