net: dwc_eth_qos: implement phy reg and max-speed for stm32

Add management of property "reg" to configure @ of phy and
also "max-speed" property to specify maximum speed in Mbit/s
supported by the device

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>