configs: tqma6: switch to DM_SERIAL

Usage without DM_SERIAL is deprecated. Fix this.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
diff --git a/configs/tqma6s_mba6_mmc_defconfig b/configs/tqma6s_mba6_mmc_defconfig
index 9400c64..84eb98f 100644
--- a/configs/tqma6s_mba6_mmc_defconfig
+++ b/configs/tqma6s_mba6_mmc_defconfig
@@ -61,4 +61,4 @@
 CONFIG_DM_REGULATOR_PFUZE100=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
-# CONFIG_SPECIFY_CONSOLE_INDEX is not set
+CONFIG_DM_SERIAL=y