Separate YAML files validation

We borrow the validate.py script from the SystemReady scripts
repository[1] to validate our YAML files against schemas.

We deprecate the `--validate-config' and the `--validate-seq-db'
command line options of the parser.

We adapt the `make check' commands.

[1]: https://gitlab.arm.com/systemready/systemready-scripts



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