This patch enables these configs:

+CONFIG_SERIAL_DEV_BUS=y
+CONFIG_SERIAL_DEV_CTRL_TTYPORT=y

As example, a bluetooth device connected to UART port can be supported by
this.
Signed-off-by: NGuodong Xu <guodong.xu@linaro.org>
Signed-off-by: NWei Xu <xuwei5@hisilicon.com>