Tested with:
Copy /lib/systemd/system/serial-getty@.service to /etc/systemd/system
cp /lib/systemd/system/serial-getty@.service /etc/systemd/system
Navigate to /etc/systemd/system/
cd /etc/systemd/system/
Rename serial-getty@.service
mv serial-getty@.service serial-getty@<console_device>.service
Edit serial-getty@ttymxc1.service with e.g. nano editor
nano serial-getty@ttymxc1.service
Modify the line that begins with ExecStart as follows
ExecStart=-/sbin/agetty -a <username> <console_device> <buadrate> %I $TERM
systemctl enable serial-getty@<console_device>.service