dt-bindings: clk: stm32h7: fix clock-cell size

The clock-cell size is 1 on stm32h7 plaform.
Signed-off-by: NGabriel Fernandez <gabriel.fernandez@st.com>
Fixes: 3e4d618b ("clk: stm32h7: Add stm32h743 clock driver")
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/clock/st,stm32h7-rcc.txt

@@ -32,7 +32,7 @@ Example:
 		compatible = "st,stm32h743-rcc", "st,stm32-rcc";
 		reg = <0x58024400 0x400>;
 		#reset-cells = <1>;
-		#clock-cells = <2>;
+		#clock-cells = <1>;
 		clocks = <&clk_hse>, <&clk_lse>, <&clk_i2s_ckin>;
 
 		st,syscfg = <&pwrcfg>;