scripts: extract_dts_inlcudes: look up compatible field in parents

Modify get_compat function to look for 'compatible' property
in parent nodes if not available at node level.
Since this operation is quite common, this allows some code
factorization. As a consequence, get_compat takes 'node_address'
as argument instead of 'node'.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>