dt: add a binding review checklist

I originally intended this to be a brief list of items to check when
reviewing a new DT binding. However, I appear to have become slightly
distracted and added in a bunch of background/justification verbiage
too!

Do people think this is going the right direction? Should I continue in
the current style and add more sections to the document, or strip it back
to be a much more plain list, perhaps something like:
https://fedoraproject.org/wiki/Packaging:ReviewGuidelines
(which is where I got the idea for this)

Signed-off-by: Stephen Warren <swarren@nvidia.com>