arm64: dts: imx8mm-u-boot.dtsi: explicitly add spl filename

Explicitly add SPL aka u-boot-spl.bin filename.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/dts/imx8mm-u-boot.dtsi b/arch/arm/dts/imx8mm-u-boot.dtsi
index 770282d..8feb0f2 100644
--- a/arch/arm/dts/imx8mm-u-boot.dtsi
+++ b/arch/arm/dts/imx8mm-u-boot.dtsi
@@ -36,6 +36,7 @@
 
 		u-boot-spl {
 			align-end = <4>;
+			filename = "u-boot-spl.bin";
 		};
 
 		blob_1: blob-ext@1 {