dm: tegra: Enable driver model for serial

Use driver model for serial ports.

Since Tegra now uses driver model for serial, adjust the definition of
V_NS16550_CLK so that it is clear that this is only used for SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 5ae6416..853a8c6 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -38,6 +38,7 @@
 obj-$(CONFIG_FSL_LPUART) += serial_lpuart.o
 obj-$(CONFIG_MXS_AUART) += mxs_auart.o
 obj-$(CONFIG_ARC_SERIAL) += serial_arc.o
+obj-$(CONFIG_TEGRA_SERIAL) += serial_tegra.o
 
 ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_USB_TTY) += usbtty.o