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/drivers/reset/Makefile b/drivers/reset/Makefile
index 2eb639e..d99a78c 100644
--- a/drivers/reset/Makefile
+++ b/drivers/reset/Makefile
@@ -32,4 +32,4 @@
 obj-$(CONFIG_RESET_ZYNQMP) += reset-zynqmp.o
 obj-$(CONFIG_RESET_DRA7) += reset-dra7.o
 obj-$(CONFIG_RESET_AT91) += reset-at91.o
-obj-$(CONFIG_$(SPL_TPL_)RESET_JH7110) += reset-jh7110.o
+obj-$(CONFIG_$(PHASE_)RESET_JH7110) += reset-jh7110.o