arm: ls1021atwr: Convert to driver model and enable serial support

Convert ls1021atwr_nor to driver model support. As a start, enable
ns16550 serial port driver.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ls1021atwr_nor_defconfig b/configs/ls1021atwr_nor_defconfig
index aa874fd..2b79443 100644
--- a/configs/ls1021atwr_nor_defconfig
+++ b/configs/ls1021atwr_nor_defconfig
@@ -1,6 +1,10 @@
 CONFIG_ARM=y
 CONFIG_TARGET_LS1021ATWR=y
+CONFIG_DM_SERIAL=y
+CONFIG_DEFAULT_DEVICE_TREE="ls1021a-twr"
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_CONTROL=y
+CONFIG_DM=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y