of: properties of reserved-memory nodes

The reusable and the no-map property are mutually exclusive.
Clarify this in the documentation.

Acked-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Link: https://lore.kernel.org/r/20200910162020.3927-1-xypron.glpk@gmx.de
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt

@@ -64,6 +64,9 @@ reusable (optional) - empty property
       system can use that region to store volatile or cached data that
       can be otherwise regenerated or migrated elsewhere.
 
+A node must not carry both the no-map and the reusable property as these are
+logically contradictory.
+
 Linux implementation note:
 - If a "linux,cma-default" property is present, then Linux will use the
   region for the default pool of the contiguous memory allocator.