ARM: dts: stm32: Add compatibles for syscon for stm32f746

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/stm32f746.dtsi

@@ -495,8 +495,8 @@ sdio1: sdio1@40012c00 {
 			status = "disabled";
 		};
 
-		syscfg: system-config@40013800 {
-			compatible = "syscon";
+		syscfg: syscon@40013800 {
+			compatible = "st,stm32-syscfg", "syscon";
 			reg = <0x40013800 0x400>;
 		};
 
@@ -563,7 +563,7 @@ pwm {
 		};
 
 		pwrcfg: power-config@40007000 {
-			compatible = "syscon";
+			compatible = "st,stm32-power-config", "syscon";
 			reg = <0x40007000 0x400>;
 		};