ARM: dts: stm32: Add compatibles for syscon for stm32mp151

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/stm32mp151.dtsi

···
 		};
 
 		pwr_mcu: pwr_mcu@50001014 {
-			compatible = "syscon";
+			compatible = "st,stm32mp151-pwr-mcu", "syscon";
 			reg = <0x50001014 0x4>;
 		};