patman: By default don't pass "--no-tree" to checkpatch for linux

When you pass "--no-tree" to checkpatch it disables some extra checks
that are important for Linux. Specifically I want checks like:

  warning: DT compatible string "boogie,woogie" appears un-documented
  check ./Documentation/devicetree/bindings/

Let's make the default for Linux to _not_ pass --no-tree. We'll have a
config option and command line flag to override.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>