dt-bindings: stm32: syscon: Add clock support

STM32 system configuration controller registers needs to be clocked.
Document clock support on stm32-syscon.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/arm/stm32/stm32-syscon.txt

@@ -5,10 +5,12 @@ Properties:
                  - " st,stm32mp157-syscfg " - for stm32mp157 based SoCs,
                  second value must be always "syscon".
    - reg : offset and length of the register set.
+   - clocks: phandle to the syscfg clock
 
  Example:
          syscfg: syscon@50020000 {
                  compatible = "st,stm32mp157-syscfg", "syscon";
                  reg = <0x50020000 0x400>;
+                 clocks = <&rcc SYSCFG>;
          };