phy: stm32-usbphyc: stm32: usbphyc: add protection on phy sub-node

Add protection on presence and order of the phy node sub node
by using the mandatory reg information.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>