global: Rename SPL_ to XPL_

Use XPL_ as the symbol to indicate an SPL build. This means that SPL_ is
no-longer set.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/power/domain/Makefile b/drivers/power/domain/Makefile
index 2daab73..110646c 100644
--- a/drivers/power/domain/Makefile
+++ b/drivers/power/domain/Makefile
@@ -3,7 +3,7 @@
 # Copyright (c) 2016, NVIDIA CORPORATION.
 #
 
-obj-$(CONFIG_$(SPL_)POWER_DOMAIN) += power-domain-uclass.o
+obj-$(CONFIG_$(XPL_)POWER_DOMAIN) += power-domain-uclass.o
 obj-$(CONFIG_APPLE_PMGR_POWER_DOMAIN) += apple-pmgr.o
 obj-$(CONFIG_BCM6328_POWER_DOMAIN) += bcm6328-power-domain.o
 obj-$(CONFIG_IMX8_POWER_DOMAIN) += imx8-power-domain-legacy.o imx8-power-domain.o