checkpatch: DT bindings should be a separate patch

Devicetree bindings should be their own patch as documented in
Documentation/devicetree/bindings/submitting-patches.txt section I.1.
This is because bindings are logically independent from a driver
implementation, they have a different maintainer (even though they often
are applied via the same tree), and it makes for a cleaner history in the
DT only tree created with git-filter-branch.

[robh@kernel.org: add doc pointer to warning, simplify logic]
  Link: http://lkml.kernel.org/r/20180810170513.26284-1-robh@kernel.org
[robh@kernel.org: v3]
  Link: http://lkml.kernel.org/r/20180810225049.20452-1-robh@kernel.org
Link: http://lkml.kernel.org/r/20180809205032.22205-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>