binman: Use a reference for binman symbols docs

Several etypes have this reference in their documentation. Now that we are
using rST, link to the section directly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
diff --git a/tools/binman/binman.rst b/tools/binman/binman.rst
index 2899e1c..ef3e5a6 100644
--- a/tools/binman/binman.rst
+++ b/tools/binman/binman.rst
@@ -487,6 +487,7 @@
 since it is assumed that images are XIP and the offsets already include the
 address.
 
+.. _binman_fdt:
 
 Access to binman entry offsets at run time (fdt)
 ------------------------------------------------