dm: serial: Allow driver-model serial to be disabled for TPL

Add separate enable/disable controls for driver-model serial. While this
is generally enabled in SPL it may not be in TPL, since serial output can
be obtained with the debug UART with minimal code size.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 4ed4bba..8ba15ce 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -6,7 +6,7 @@
 #
 
 ifdef CONFIG_DM_SERIAL
-obj-y += serial-uclass.o
+obj-$(CONFIG_$(SPL_TPL_)DM_SERIAL) += serial-uclass.o
 obj-$(CONFIG_PL01X_SERIAL) += serial_pl01x.o
 else
 obj-y += serial.o