ARM: dts: stm32: Add generic SoM compatible to STM32MP15xx DH electronics DHSOM

Add generic SoM compatible string into machine compatible string
for all STM32MP15xx based DH electronics DHSOM. This way, common
board code can match on this compatible. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/arch/arm/dts/stm32mp15xx-dhcom-pdk2.dts b/arch/arm/dts/stm32mp15xx-dhcom-pdk2.dts
index e2e01e2..b2e450a 100644
--- a/arch/arm/dts/stm32mp15xx-dhcom-pdk2.dts
+++ b/arch/arm/dts/stm32mp15xx-dhcom-pdk2.dts
@@ -11,5 +11,7 @@
 
 / {
 	model = "STMicroelectronics STM32MP15xx DHCOM Premium Developer Kit (2)";
-	compatible = "dh,stm32mp15xx-dhcom-pdk2", "st,stm32mp15x";
+	compatible = "dh,stm32mp15xx-dhcom-pdk2",
+		     "dh,stm32mp15xx-dhcom-som",
+		     "st,stm32mp15x";
 };