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/cpu.c b/arch/mips/mach-mtmips/cpu.c
index 243938a..982a588 100644
--- a/arch/mips/mach-mtmips/cpu.c
+++ b/arch/mips/mach-mtmips/cpu.c
@@ -21,7 +21,7 @@
 	return 0;
 }
 
-#ifndef CONFIG_SPL_BUILD
+#ifndef CONFIG_XPL_BUILD
 static int last_stage_init(void)
 {
 	void *src, *dst;