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 <[e-mail]>
Signed-off-by: Alexandre Torgue <[e-mail]>

arch/arm/boot/dts/stm32f746.dtsi

···
 			status = "disabled";
 		};
 
-		syscfg: system-config@40013800 {
-			compatible = "syscon";
+		syscfg: syscon@40013800 {
+			compatible = "st,stm32-syscfg", "syscon";
 			reg = <0x40013800 0x400>;
 		};
 
···
 		};
 
 		pwrcfg: power-config@40007000 {
-			compatible = "syscon";
+			compatible = "st,stm32-power-config", "syscon";
 			reg = <0x40007000 0x400>;
 		};