engicam: imx6: migrate to DM_SERIAL

Add the needed DT overrides and configs to enable UART in SPL.

Cc: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Tested-by: Suniel Mahesh <sunil@amarulasolutions.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>