arm: dts: imx8m: update binman ddr firmware node name

We are migrating to use binman symbols, the current names are
inconsistent across different boards, so unify them.

Also add `type = "blob-ext";`, since the new names are not valid binman
types.

Tested-by: Tim Harvey <tharvey@gateworks.com> #imx8m[m,n,p]-venice
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
[Alper: Edit commit message]
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.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 6e37622..b8df6f7 100644
--- a/arch/arm/dts/imx8mn-var-som-symphony-u-boot.dtsi
+++ b/arch/arm/dts/imx8mn-var-som-symphony-u-boot.dtsi
@@ -130,25 +130,25 @@
 			filename = "u-boot-spl.bin";
 		};
 
-		1d-imem {
+		ddr-1d-imem-fw {
 			filename = "ddr4_imem_1d.bin";
 			size = <0x8000>;
 			type = "blob-ext";
 		};
 
-		1d_dmem {
+		ddr-1d-dmem-fw {
 			filename = "ddr4_dmem_1d.bin";
 			size = <0x4000>;
 			type = "blob-ext";
 		};
 
-		2d_imem {
+		ddr-2d-imem-fw {
 			filename = "ddr4_imem_2d.bin";
 			size = <0x8000>;
 			type = "blob-ext";
 		};
 
-		2d_dmem {
+		ddr-2d-dmem-fw {
 			filename = "ddr4_dmem_2d.bin";
 			size = <0x4000>;
 			type = "blob-ext";