device_table_dev.txt: Increase /dev/ttymxc count

Since debug uart on imx6 sabreauto board is ttymxc3.

Signed-off-by: Abdul Hussain <hussain.abdul@outlook.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>