ARM: dts: sti: move usb picophy nodes out of soc in stih418.dtsi

Move the usb2_picophy1 and usb2_picophy2 nodes out of the soc section.
Since they are controlled via syscfg, there is no reg property needed,
which is required when having the node within the soc section.

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>