drivers: Use CONFIG_XPL_BUILD instead of CONFIG_SPL_BUILD

Use the new symbol to refer to any 'SPL' build, including TPL and VPL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/ddr/imx/imx9/Makefile b/drivers/ddr/imx/imx9/Makefile
index 9403f98..6c95060 100644
--- a/drivers/ddr/imx/imx9/Makefile
+++ b/drivers/ddr/imx/imx9/Makefile
@@ -4,7 +4,7 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_XPL_BUILD
 obj-$(CONFIG_IMX9_DRAM) += ddr_init.o
 obj-y += ../phy/
 endif