imx6: disable UART dma

imx6: disable UART dma

Fixes these prints on boot:

[   13.785600] imx-uart 2020000.serial: DMA transaction error.
[   13.793134] imx-uart 2020000.serial: DMA transaction error.
[   13.798721] imx-uart 2020000.serial: DMA transaction error.

Based on following upstream patch by Tim Harvey (Gateworks):

https://github.com/Gateworks/openwrt/commit/80a01b6582f94c4547f39d3a25e0a1e9b6eb9877

TX complete DMA messages are getting missed.
This is also currently an issue in mainline.
For now we will disable DMA in serial/imx.c.

This resolves an issue encountered with RS485 transmit.

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