actions: Add some additional labels for devicetree

- Handle name change of labels from 'Device Tree' to Devicetree
- Split out things between "Devicetree" (as a catchall),
  "Devicetree Tooling" (for the scripts) and
  "Devicetree Binding" (for bindings)

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>