dt-bindings: phy: phy-stm32-usbphyc: add #clock-cells property

usbphyc provides a unique clock called ck_usbo_48m.
STM32 USB OTG needs a 48Mhz clock (utmifs_clk48) for Full-Speed operation.
ck_usbo_48m is a possible parent clock for USB OTG 48Mhz clock.

ck_usbo_48m is available as soon as the PLL is enabled.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210208114659.15269-2-amelie.delaunay@foss.st.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>