dt-bindings: submitting-patches: add guidance on patch content and subject

Clarify that binding patches should also include include/dt-bindings/*
as part of them. The binding doc defines the ABI and the includes are
part of that. Add some details on the preferred subject prefix and
contents.
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/submitting-patches.txt

@@ -6,7 +6,14 @@ I. For patch submitters
   0) Normal patch submission rules from Documentation/process/submitting-patches.rst
      applies.
 
-  1) The Documentation/ portion of the patch should be a separate patch.
+  1) The Documentation/ and include/dt-bindings/ portion of the patch should
+     be a separate patch. The preferred subject prefix for binding patches is:
+
+       "dt-bindings: <binding dir>: ..."
+
+     The 80 characters of the subject are precious. It is recommended to not
+     use "Documentation" or "doc" because that is implied. All bindings are
+     docs. Repeating "binding" again should also be avoided.
 
   2) Submit the entire series to the devicetree mailinglist at