dm: imx: serial: Support DTE mode when using driver model

The MXC UART IP can be run in DTE or DCE mode. This depends on the
board wiring and the pinmux used and hence is board specific. This
extends platform data with a new field to choose wheather DTE
mode shall be used.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>