serial: ns16550: Allow serial to enabled/disabled in SPL

At present this driver uses the wrong condition for including the code and
drivers in SPL/TPL. Update it so that the code is only included if
DM_SERIAL is enabled for SPL/TPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed