dm: Add a uclass for serial devices

Serial devices support simple byte input/output and a few operations to find
out whether data is available. Add a basic uclass for serial devices to be
used by drivers that are converted to driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 571c18f..4720e1d 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -5,7 +5,11 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+ifdef CONFIG_DM_SERIAL
+obj-y += serial-uclass.o
+else
 obj-y += serial.o
+endif
 
 obj-$(CONFIG_ALTERA_UART) += altera_uart.o
 obj-$(CONFIG_ALTERA_JTAG_UART) += altera_jtag_uart.o