arm: kirkwood: Enable Debug UART for Zyxel NSA310S

It's useful to enable Debug UART for future DM Serial regression tests
for Kirkwood boards.

Also, see background discussion in this thread:
https://lists.denx.de/pipermail/u-boot/2023-March/512010.html



Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>