ARM: dts: stm32: Use DT alias for the configuration EEPROM

Use DT /aliases node to establish a stable phandle to the configuration
EEPROM. This permits the configuration EEPROM to be moved e.g. to a
different address or a different bus. Adjust the board code to handle
new phandle lookup.

Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>