arm: dts: imx8mn-u-boot: fix DDR3 only support

In case the CONFIG_IMX8M_LPDDR4 and CONFIG_IMX8M_DDR4 options are both
disabled (i. e. BSH boards), binmain fails because DDR4 bin files are
missing.

Fixes: 93c4c0e4dd1e75 ("arm: dts: imx8mn-u-boot: Create common imx8mn-u-boot.dtsi")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
diff --git a/arch/arm/dts/imx8mn-u-boot.dtsi b/arch/arm/dts/imx8mn-u-boot.dtsi
index 95f45ad..ec53390 100644
--- a/arch/arm/dts/imx8mn-u-boot.dtsi
+++ b/arch/arm/dts/imx8mn-u-boot.dtsi
@@ -105,10 +105,11 @@
 			align-end = <4>;
 		};
 
+#if defined(CONFIG_IMX8M_LPDDR4) || defined(CONFIG_IMX8M_DDR4)
 		ddr-2d-imem-fw {
 #ifdef CONFIG_IMX8M_LPDDR4
 			filename = "lpddr4_pmu_train_2d_imem.bin";
-#elif CONFIG_IMX8M_DDR4
+#else
 			filename = "ddr4_imem_2d.bin";
 #endif
 			type = "blob-ext";
@@ -118,12 +119,13 @@
 		ddr-2d-dmem-fw {
 #ifdef CONFIG_IMX8M_LPDDR4
 			filename = "lpddr4_pmu_train_2d_dmem.bin";
-#elif CONFIG_IMX8M_DDR4
+#else
 			filename = "ddr4_dmem_2d.bin";
 #endif
 			type = "blob-ext";
 			align-end = <4>;
 		};
+#endif
 	};
 
 	spl {