dt-bindings: Add running yamllint to dt_binding_check

Add a yamllint config file and support for running yamllint on DT
binding schema files. This runs on the whole tree as yamllint is Python
and suffers from Python's slow startup times.

Users can run on individual files doing:

yamllint -c Documentation/devicetree/bindings/.yamllint <binding file>

Link: https://lore.kernel.org/r/20201009170557.168785-1-robh@kernel.org


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