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/arch/arm/mach-stm32mp/stm32mp1/Makefile b/arch/arm/mach-stm32mp/stm32mp1/Makefile
index 5ae1a53..db160c2 100644
--- a/arch/arm/mach-stm32mp/stm32mp1/Makefile
+++ b/arch/arm/mach-stm32mp/stm32mp1/Makefile
@@ -15,5 +15,5 @@
 obj-$(CONFIG_ARMV7_PSCI) += psci.o
 endif
 
-obj-$(CONFIG_$(SPL_)STM32MP15_PWR) += pwr_regulator.o
+obj-$(CONFIG_$(XPL_)STM32MP15_PWR) += pwr_regulator.o
 obj-$(CONFIG_OF_SYSTEM_SETUP) += fdt.o