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-picoitx.dts b/arch/arm/dts/stm32mp15xx-dhcom-picoitx.dts
index 06770b4..3e90810 100644
--- a/arch/arm/dts/stm32mp15xx-dhcom-picoitx.dts
+++ b/arch/arm/dts/stm32mp15xx-dhcom-picoitx.dts
@@ -11,5 +11,7 @@
 
 / {
 	model = "DH Electronics STM32MP15xx DHCOM PicoITX";
-	compatible = "dh,stm32mp15xx-dhcom-picoitx", "st,stm32mp1xx";
+	compatible = "dh,stm32mp15xx-dhcom-picoitx",
+		     "dh,stm32mp15xx-dhcom-som",
+		     "st,stm32mp1xx";
 };