The serial port was being manually configured during SPL build,
however in preparation to allow DM in SPL, this needs to change
to be based on whether or not DM_SERIAL is enabled because, soon
the assumption that SPL means no DM may not be accurate.
Signed-off-by: NAdam Ford <aford173@gmail.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>