arm: dts: imx8mn_var_som: Set atf-bl31 blob entry type

Configure binman ATF blob entry type to use the path from the BL31
environment variable, if defined.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
diff --git a/arch/arm/dts/imx8mn-var-som-symphony-u-boot.dtsi b/arch/arm/dts/imx8mn-var-som-symphony-u-boot.dtsi
index ce47588..6e37622 100644
--- a/arch/arm/dts/imx8mn-var-som-symphony-u-boot.dtsi
+++ b/arch/arm/dts/imx8mn-var-som-symphony-u-boot.dtsi
@@ -200,7 +200,7 @@
 
 					atf_blob {
 						filename = "bl31.bin";
-						type = "blob-ext";
+						type = "atf-bl31";
 					};
 				};