Documentation: DT: document compatible string existence requirement

DT maintainers require all compatible strings used in chip or board
DTS file to be previously documented somewhere in
Documentation/devicetree/bindings, per:

http://marc.info/?l=linux-tegra&m=142201349727836&w=2

Document this requirement in the DT patch submission requirements
text file.

This second version updates the documentation to align with
Rob's comments here:

http://marc.info/?l=devicetree&m=142255654213019&w=2

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Paul Walmsley <pwalmsley@nvidia.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/submitting-patches.txt

@@ -15,6 +15,29 @@ I. For patch submitters
   3) The Documentation/ portion of the patch should come in the series before
      the code implementing the binding.
 
+  4) Any compatible strings used in a chip or board DTS file must be
+     previously documented in the corresponding DT binding text file
+     in Documentation/devicetree/bindings.  This rule applies even if
+     the Linux device driver does not yet match on the compatible
+     string.  [ checkpatch will emit warnings if this step is not
+     followed as of commit bff5da4335256513497cc8c79f9a9d1665e09864
+     ("checkpatch: add DT compatible string documentation checks"). ]
+
+  5) The wildcard "<chip>" may be used in compatible strings, as in
+     the following example:
+
+         - compatible: Must contain '"nvidia,<chip>-pcie",
+           "nvidia,tegra20-pcie"' where <chip> is tegra30, tegra132, ...
+
+     As in the above example, the known values of "<chip>" should be
+     documented if it is used.
+
+  6) If a documented compatible string is not yet matched by the
+     driver, the documentation should also include a compatible
+     string that is matched by the driver (as in the "nvidia,tegra20-pcie"
+     example above).
+
+
 II. For kernel maintainers
 
   1) If you aren't comfortable reviewing a given binding, reply to it and ask