board: lsxl: convert to DM_SERIAL

DM_SERIAL needs early malloc. The on-chip RAM is pretty tight, it's only
2kiB, with DM_SERIAL enabled, this doesn't work anymore. Fortunately for
us, we don't need the on-chip RAM because the DRAM is already
initialized before u-boot starts. Just put the early malloc area there
and use the default early malloc size.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Stefan Roese <sr@denx.de>