arm: kirkwood: Use CONFIG_SYS_NS16550 with DM_SERIAL for Kirkwood boards

CONFIG_SYS_NS16550 is required when DM_SERIAL is enabled for
Kirkwood boards.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>