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 <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp151.dtsi

@@ -1127,7 +1127,7 @@
 		};
 
 		pwr_mcu: pwr_mcu@50001014 {
-			compatible = "syscon";
+			compatible = "st,stm32mp151-pwr-mcu", "syscon";
 			reg = <0x50001014 0x4>;
 		};