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/riscv/cpu/fu540/Makefile b/arch/riscv/cpu/fu540/Makefile
index 043fb96..69759c5 100644
--- a/arch/riscv/cpu/fu540/Makefile
+++ b/arch/riscv/cpu/fu540/Makefile
@@ -3,7 +3,7 @@
 # Copyright (C) 2020 SiFive, Inc
 # Pragnesh Patel <pragnesh.patel@sifive.com>
 
-ifeq ($(CONFIG_SPL_BUILD),y)
+ifeq ($(CONFIG_XPL_BUILD),y)
 obj-y += spl.o
 else
 obj-y += dram.o