Documentation: DT: document compatible string existence requirement

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>