dt: submitting-patches: clarify that DT maintainers are to be cced on bindings

The exact steps provided for submitting binding patches can be read
as requiring the bindings to be sent only to the devicetree@vger.kernel.org
list. Since the DT maintainers would like to be Cced on any binding
submissions, make this requirement explicit in step 2.

Signed-off-by: Matt Porter <mporter@konsulko.com>
Signed-off-by: Rob Herring <robh@kernel.org>