dm: Add docs to explain how to enable DM_SERIAL for a board

This is an attempt to cover the common cases found when enabling driver
model for serial on a new board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>