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>

system/device_table_dev.txt

@@ -35,7 +35,7 @@
 /dev/ttySC	c	666	0	0	204	8	0	1	8
 /dev/ttyPSC	c	666	0	0	204	148	0	1	4
 /dev/ttyUL	c	666	0	0	204	187	0	1	4
-/dev/ttymxc	c	666	0	0	207	16	0	1	3
+/dev/ttymxc	c	666	0	0	207	16	0	1	4
 
 # Input stuff
 /dev/input		d	755	0	0	-	-	-	-	-