ARM: dts: sti: ensure unique unit-address in stih407-clock

Move quadfs and a9-mux clocks nodes into clockgen nodes so
that they can get the reg property from the parent node and
ensure only one node has the address.

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>