dt-bindings: serial: imx: Document optional DMA properties

The UART IP can be connected to DMA engine, document the properties in DT bindings.
Update example to match Linux arch/arm/boot/dts/imx51.dtsi .

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230219142250.10176-1-marex@denx.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>