dt-bindings: serial: fsl-imx-uart: Remove 'fsl,irda-mode' property

'fsl,irda-mode' property has been removed since commit afe9cbb1
("serial: imx: drop support for IRDA"), so remove it from the binding
document.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>