net: dwc_eth_qos: implement reset-gpios for stm32

Add management of property "reset-gpios" in the node identified by
"phy-handle" to configure any GPIO used to reset the PHY.

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