Documentation: dt-bindings: Explain order in patch series

When posting a patch series that includes both code implementing a
Device Tree binding and its associated documentation, the DT docs
should come in the series before the implementation.

This not only avoids checkpatch.pl to complain about undocumented
bindings but also makes the review process easier.

Document this convention since it may not be obvious.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>