mkimage: Support adding device tree files to a FIT

To make the auto-FIT feature useful we need to be able to provide a list of
device tree files on the command line for mkimage to add into the FIT. Add
support for this feature.

So far there is no support for hashing or verified boot using this method.
For those cases, a .its file must still be provided.

Signed-off-by: Simon Glass <sjg@chromium.org>