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/mips/mach-mtmips/Makefile b/arch/mips/mach-mtmips/Makefile
index 19f1e07..2f35b1a 100644
--- a/arch/mips/mach-mtmips/Makefile
+++ b/arch/mips/mach-mtmips/Makefile
@@ -5,7 +5,7 @@
 ifneq ($(CONFIG_SOC_MT7621),y)
 obj-y += ddr_init.o
 obj-y += ddr_cal.o
-obj-$(CONFIG_SPL_BUILD) += spl.o
+obj-$(CONFIG_XPL_BUILD) += spl.o
 endif
 
 obj-$(CONFIG_SOC_MT7620) += mt7620/