check-sr-results: download bindings and run dt-validate

Add a verification of the Devicetree blob with dt-validate.

We download, extract and cache the Devicetree bindings automatically.

We add a `--dt-validate' command line option to specify the dt-validate
command, and a `--linux-url' command line option to allow specifying the
URL of the Linux kernel tarball to download.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>