scripts: dts: support multiple binding dirs in new scripts

gen_defines.py and edtlib.py were recently added in
commit 62d57414 ("dts: Add new DTS/binding parser").

The old extract_dts_includes.py script allowed for multiple
dts bindings dirs.  Let's add that functionality to the new
scripts as well.

Signed-off-by: Michael Scott <mike@foundries.io>