dm: doc: add documentation for pre-reloc properties in SPL and TPL

Add documentation for the pre-reloc property in SPL and TPL device-tree:
- u-boot,dm-pre-proper
- u-boot,dm-pre-reloc
- u-boot,dm-spl
- u-boot,dm-tpl

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>