global: Rename SPL_TPL_ to PHASE_

Use PHASE_ as the symbol to select a particular XPL build. This means
that SPL_TPL_ is no-longer set.

Update the comment in bootstage to refer to this symbol, instead of
SPL_

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/cpu/armv8/Makefile b/arch/arm/cpu/armv8/Makefile
index c5f568e..8747d2e 100644
--- a/arch/arm/cpu/armv8/Makefile
+++ b/arch/arm/cpu/armv8/Makefile
@@ -6,7 +6,7 @@
 extra-y	:= start.o
 
 obj-y	+= cpu.o
-ifndef CONFIG_$(SPL_TPL_)TIMER
+ifndef CONFIG_$(PHASE_)TIMER
 obj-$(CONFIG_SYS_ARCH_TIMER) += generic_timer.o
 endif
 ifndef CONFIG_$(XPL_)SYS_DCACHE_OFF