dm: doc: Fix serial howto u-boot,dm-pre-reloc typo

In a couple of places the document says u-boot,pre-reloc but all
examples show u-boot,dm-pre-reloc, use the latter consistently.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed