ARM: dts: stm32: Add compatibles for syscon for stm32f426

Syscon nodes needs at least 2 compatibles to be compliant why yaml documentation.
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32f429.dtsi

@@ -597,8 +597,8 @@ spi4: spi@40013400 {
 			status = "disabled";
 		};
 
-		syscfg: system-config@40013800 {
-			compatible = "syscon";
+		syscfg: syscon@40013800 {
+			compatible = "st,stm32-syscfg", "syscon";
 			reg = <0x40013800 0x400>;
 		};
 
@@ -688,7 +688,7 @@ spi6: spi@40015400 {
 		};
 
 		pwrcfg: power-config@40007000 {
-			compatible = "syscon";
+			compatible = "st,stm32-power-config", "syscon";
 			reg = <0x40007000 0x400>;
 		};