imx6: disable dma on uart

imx6: disable dma on uart

When this target got updated to 4.14, this patch got removed to
re-evaluate if it was still needed.

Extensive testing now shows this issue is still present.
Let's re-add the patch to fix it for now.

As the uart bus is very low bandwidth .. performance impact is negligible.

Boot log:

[   22.513051] imx-uart 2020000.serial: DMA transaction error.
[   22.522721] imx-uart 2020000.serial: DMA transaction error.

As a sidenote:
The patch mentiones an issue with RS485, but the bootlog
errors above were recorded with the uart ports in standard RS232 mode.

Compile/Run-tested on imx6/GW5200

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>