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

Documentation/devicetree/bindings/phy/phy-stm32-usbphyc.yaml

@@ -51,6 +51,10 @@ properties:
   vdda1v8-supply:
     description: regulator providing 1V8 power supply to the PLL block
 
+  '#clock-cells':
+    description: number of clock cells for ck_usbo_48m consumer
+    const: 0
+
 #Required child nodes:
 
 patternProperties:
@@ -120,6 +124,7 @@ examples:
         vdda1v8-supply = <&reg18>;
         #address-cells = <1>;
         #size-cells = <0>;
+        #clock-cells = <0>;
 
         usbphyc_port0: usb-phy@0 {
             reg = <0>;