dm: sunxi: Add support for serial using driver model

Add a driver for the designware serial UART used on sunxi. This just
redirects to the normal ns16550 driver.

Add a stdout-path to the device tree so that the correct UART is chosen.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>