arm64: defconfig: enable support for serial port connected device

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: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>