ARM: dts: sti: update flexgen compatible within stih407-clock

With the introduction of new flexgen compatible within the clk-flexgen
driver, remove the clock-output-names entry from the flexgen nodes
and set the new proper compatible corresponding.

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>