dt: dt-extract-compatibles: Handle cfile arguments in generator function

Move the handling of the cfile arguments to a separate generator
function to avoid redundancy.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20230828211424.2964562-2-nfraprado@collabora.com
Signed-off-by: Rob Herring <robh@kernel.org>