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-mediatek/Makefile b/arch/arm/mach-mediatek/Makefile
index 46bdab8..3d9e468 100644
--- a/arch/arm/mach-mediatek/Makefile
+++ b/arch/arm/mach-mediatek/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier:	GPL-2.0
 
 obj-y	+= cpu.o
-obj-$(CONFIG_SPL_BUILD)	+= spl.o
+obj-$(CONFIG_XPL_BUILD)	+= spl.o
 
 obj-$(CONFIG_MT8512) += mt8512/
 obj-$(CONFIG_TARGET_MT7622) += mt7622/