arch: 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/arch/arm/mach-imx/mx6/Makefile b/arch/arm/mach-imx/mx6/Makefile
index 7ea8f91..da461b0 100644
--- a/arch/arm/mach-imx/mx6/Makefile
+++ b/arch/arm/mach-imx/mx6/Makefile
@@ -7,7 +7,7 @@
 
 obj-y	:= soc.o clock.o
 obj-$(CONFIG_IMX_MODULE_FUSE) += module_fuse.o
-obj-$(CONFIG_SPL_BUILD)	     += ddr.o
+obj-$(CONFIG_XPL_BUILD)	     += ddr.o
 obj-$(CONFIG_MP)             += mp.o
 obj-$(CONFIG_MX6UL_LITESOM)  += litesom.o
 obj-$(CONFIG_MX6UL_OPOS6UL)  += opos6ul.o