dt-bindings: Add a guide of do's and don't's for writing bindings

Devicetree binding reviews have a lot of repeated review comments. Much
of the guidelines aren't written down. This list of do's and don't's is
by no means an exhaustive guide for how to write bindings, but at least
the "rules" are written down in some form.

Signed-off-by: Rob Herring <robh@kernel.org>