dt-bindings: net: stmmac: add phys config properties

Add properties to support all Phy config
 PHY_MODE	(MII,GMII, RMII, RGMII) and in normal, PHY wo crystal (25Mhz),
 PHY wo crystal (50Mhz), No 125Mhz from PHY config.

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>