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/firmware/Makefile b/drivers/firmware/Makefile
index 7ce83d7..8b979f6 100644
--- a/drivers/firmware/Makefile
+++ b/drivers/firmware/Makefile
@@ -1,5 +1,5 @@
 obj-$(CONFIG_FIRMWARE)		+= firmware-uclass.o
-obj-$(CONFIG_$(SPL_)ARM_PSCI_FW)	+= psci.o
+obj-$(CONFIG_$(XPL_)ARM_PSCI_FW)	+= psci.o
 obj-$(CONFIG_TI_SCI_PROTOCOL)	+= ti_sci.o
 obj-$(CONFIG_SANDBOX)		+= firmware-sandbox.o
 obj-$(CONFIG_ZYNQMP_FIRMWARE)	+= firmware-zynqmp.o