ARM: dts: sti: add the spinor controller node within stih407-family

The STiH407 family (and further versions STiH410/STiH418) embedded
a serial flash controller allowing fast access to SPI-NOR.
This commit adds the corresponding node, relying on the st-spi-fsm
drivers.

Signed-off-by: Alain Volmat <avolmat@me.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>