ARM: mvebu: a38x: Add missing SPDX license identfier

mv_ddr_build_message.c is generated in Marvell's standalone mv_ddr code.
When imported into u-boot we need to add the appropriate SPDX tag and
re-format it slightly.

Signed-off-by: Chris Packham <judge.packham@gmail.com>

drivers/ddr/marvell/a38x/mv_ddr_build_message.c

-const char mv_ddr_build_message[] = "";       const char mv_ddr_version_string[] = "mv_ddr: mv_ddr-armada-17.10.4";
+// SPDX-License-Identifier: GPL-2.0
+const char mv_ddr_build_message[] = "";
+const char mv_ddr_version_string[] = "mv_ddr: mv_ddr-armada-17.10.4";