dm: serial: Add driver model support for ns16550

Add driver model support so that ns16550 can support operation both with
and without driver model.

The driver needs a clock frequency so cannot stand alone unfortunately. The
clock frequency must be provided by a separate driver.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 4720e1d..5ae6416 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -9,6 +9,7 @@
 obj-y += serial-uclass.o
 else
 obj-y += serial.o
+obj-$(CONFIG_SYS_NS16550_SERIAL) += serial_ns16550.o
 endif
 
 obj-$(CONFIG_ALTERA_UART) += altera_uart.o
@@ -20,7 +21,6 @@
 obj-$(CONFIG_OPENCORES_YANU) += opencores_yanu.o
 obj-$(CONFIG_SYS_NS16550) += ns16550.o
 obj-$(CONFIG_S5P) += serial_s5p.o
-obj-$(CONFIG_SYS_NS16550_SERIAL) += serial_ns16550.o
 obj-$(CONFIG_IMX_SERIAL) += serial_imx.o
 obj-$(CONFIG_KS8695_SERIAL) += serial_ks8695.o
 obj-$(CONFIG_MAX3100_SERIAL) += serial_max3100.o