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>
diff --git a/doc/README.TPL b/doc/README.TPL
index 980debe..c94129f 100644
--- a/doc/README.TPL
+++ b/doc/README.TPL
@@ -34,6 +34,10 @@
 is set. Source files can be compiled for TPL with options choosed in the
 board config file.
 
+TPL use a small device tree (u-boot-tpl.dtb), containing only the nodes with
+the pre-relocation properties: 'u-boot,dm-pre-reloc' and 'u-boot,dm-tpl'
+(see README.SPL for details).
+
 For example:
 
 spl/Makefile: