ARM: dts: stm32: Add clock on stm32mp157c syscfg

STM32 syscfg needs a clock to access registers.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp157c.dtsi

@@ -918,6 +918,7 @@ exti: interrupt-controller@5000d000 {
 		syscfg: syscon@50020000 {
 			compatible = "st,stm32mp157-syscfg", "syscon";
 			reg = <0x50020000 0x400>;
+			clocks = <&rcc SYSCFG>;
 		};
 
 		lptimer2: timer@50021000 {