Documentation: dt-bindings: Explain order in patch series

When posting a patch series that includes both code implementing a
Device Tree binding and its associated documentation, the DT docs
should come in the series before the implementation.

This not only avoids checkpatch.pl to complain about undocumented
bindings but also makes the review process easier.

Document this convention since it may not be obvious.
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/devicetree/bindings/submitting-patches.txt

@@ -12,6 +12,9 @@ I. For patch submitters
 
        devicetree@vger.kernel.org
 
+  3) The Documentation/ portion of the patch should come in the series before
+     the code implementing the binding.
+
 II. For kernel maintainers
 
   1) If you aren't comfortable reviewing a given binding, reply to it and ask