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/spi/Makefile b/drivers/spi/Makefile
index 32d7bf7..7051e2a 100644
--- a/drivers/spi/Makefile
+++ b/drivers/spi/Makefile
@@ -4,7 +4,7 @@
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
 # There are many options which enable SPI, so make this library available
-ifdef CONFIG_$(SPL_TPL_)DM_SPI
+ifdef CONFIG_$(PHASE_)DM_SPI
 obj-y += spi-uclass.o
 obj-$(CONFIG_CADENCE_QSPI) += cadence_qspi.o cadence_qspi_apb.o
 obj-$(CONFIG_CADENCE_OSPI_VERSAL) += cadence_ospi_versal.o