phy: stm32-usbphyc: usbphyc is a clock provider of ck_usbo_48m clock

ck_usbo_48m is generated by usbphyc PLL and used by OTG controller
for Full-Speed use cases with dedicated Full-Speed transceiver.

ck_usbo_48m is available as soon as the PLL is enabled.

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