arm64: dts: imx8mm-u-boot.dtsi: imx8mp-u-boot.dtsi: use atf-bl31 type

Explicitly use the atf-bl31 type for the bl31.bin atf-blob. This uses
the path from the BL31 environment variable, if defined.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
diff --git a/arch/arm/dts/imx8mm-u-boot.dtsi b/arch/arm/dts/imx8mm-u-boot.dtsi
index 3ea03a9..9f66cdb 100644
--- a/arch/arm/dts/imx8mm-u-boot.dtsi
+++ b/arch/arm/dts/imx8mm-u-boot.dtsi
@@ -109,7 +109,7 @@
 
 					atf-blob {
 						filename = "bl31.bin";
-						type = "blob-ext";
+						type = "atf-bl31";
 					};
 				};
 
diff --git a/arch/arm/dts/imx8mp-u-boot.dtsi b/arch/arm/dts/imx8mp-u-boot.dtsi
index 120c4c4..cfc352a 100644
--- a/arch/arm/dts/imx8mp-u-boot.dtsi
+++ b/arch/arm/dts/imx8mp-u-boot.dtsi
@@ -123,8 +123,9 @@
 					load = <0x970000>;
 					entry = <0x970000>;
 
-					atf_blob: blob-ext {
+					atf_blob: atf-blob {
 						filename = "bl31.bin";
+						type = "atf-bl31";
 					};
 				};