arm: mvebu: ds414: add u-boot,dm-pre-reloc to spi0

Without this U-Boot-specific property, booting on the Synology DS414
(or DS214+) fails in SPL. The spi0 DT node is not scanned, as a result
the SPI flash cannot be found.

Signed-off-by: Ezra Buehler <ezra@easyb.ch>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/arch/arm/dts/armada-xp-synology-ds414-u-boot.dtsi b/arch/arm/dts/armada-xp-synology-ds414-u-boot.dtsi
new file mode 100644
index 0000000..22fae16
--- /dev/null
+++ b/arch/arm/dts/armada-xp-synology-ds414-u-boot.dtsi
@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+&spi0 {
+	u-boot,dm-pre-reloc;
+
+	spi-flash@0 {
+		u-boot,dm-pre-reloc;
+	};
+};