imx6: fix DMA transaction errors

Following errors were seen in the past on imx6 when using serial:

[ 22.617622] imx-uart 2020000.serial: DMA transaction error.
[ 22.623228] imx-uart 2020000.serial: DMA transaction error.
[ 22.628826] imx-uart 2020000.serial: DMA transaction error.
[ 22.648951] imx-uart 2020000.serial: DMA transaction error.
[ 22.654558] imx-uart 2020000.serial: DMA transaction error.
[ 22.660156] imx-uart 2020000.serial: DMA transaction error.

Which is the reason why DMA for the serial ports
got disabled in commits:

efb362c ("imx6: disable dma on uart")
3b42410 ("imx6: disable UART dma")

As indicated on mailinglist discussion, the cause seems to be
the usage of very old SDMA firmware which is present in the soc:

[    0.624302] imx-sdma 20ec000.sdma: Direct firmware load for imx/sdma/sdma-imx6q.bin failed with error -2
[    0.624318] imx-sdma 20ec000.sdma: Falling back to user helper
[   64.531607] imx-sdma 20ec000.sdma: external firmware not found, using ROM firmware

This patch adds the new firmware binary. (2196 bytes)

It is required to embed the binary into the kernel image, as it
gets loaded very early in the boot process where the rootfs is not
available yet:

[    0.622966] imx-sdma 20ec000.sdma: loaded firmware 3.3

Extended testing shows that the DMA errors are not seen anymore
when using this newer firmware version.

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

target/linux/imx6/config-4.14

@@ -178,6 +178,8 @@ CONFIG_EXT4_FS_ENCRYPTION=y
 CONFIG_EXT4_FS_POSIX_ACL=y
 CONFIG_EXT4_FS_SECURITY=y
 CONFIG_EXTCON=y
+CONFIG_EXTRA_FIRMWARE="imx/sdma/sdma-imx6q.bin"
+CONFIG_EXTRA_FIRMWARE_DIR="firmware"
 CONFIG_FEC=y
 CONFIG_FIXED_PHY=y
 CONFIG_FIX_EARLYCON_MEM=y