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/imx8ulp/Makefile b/arch/arm/mach-imx/imx8ulp/Makefile
index 2c9938f..b478dab 100644
--- a/arch/arm/mach-imx/imx8ulp/Makefile
+++ b/arch/arm/mach-imx/imx8ulp/Makefile
@@ -6,6 +6,6 @@
 obj-y += lowlevel_init.o
 obj-y += soc.o clock.o iomux.o pcc.o cgc.o rdc.o
 
-ifeq ($(CONFIG_SPL_BUILD),y)
+ifeq ($(CONFIG_XPL_BUILD),y)
 obj-y += upower/
 endif